How reproducible:

Using a Qemu VM with Gentoo minimal ISO.
VM has 3 drive of 10GB


Steps to Reproduce:
(Line begining with # are commands executed under root)

# /etc/init.d/lvm start
 * Starting lvmetad ...
 
# rc-status
Dynamic Runlevel: needed/wanted
 lvmetad                                                          [  started  ]
 lvm                                                              [  started  ]

# lsblk -o NAME,SIZE,TYPE,FSTYPE,MOUNTPOINT
NAME   SIZE TYPE FSTYPE   MOUNTPOINT
loop0  313M loop squashfs /mnt/livecd
sda     10G disk          
sdb     10G disk          
sdc     10G disk          
sr0    347M rom  iso9660  /mnt/cdrom

# pvcreate /dev/sda /dev/sdb /dev/sdc
  Physical volume "/dev/sda" successfully created.
  Physical volume "/dev/sdb" successfully created.
  Physical volume "/dev/sdc" successfully created.

# vgcreate test /dev/sda /dev/sdb /dev/sdc
  Volume group "test" successfully created

# pvs
  PV         VG   Fmt  Attr PSize   PFree  
  /dev/sda   test lvm2 a--  <10,00g <10,00g
  /dev/sdb   test lvm2 a--  <10,00g <10,00g
  /dev/sdc   test lvm2 a--  <10,00g <10,00g

# vgs
  VG   #PV #LV #SN Attr   VSize   VFree  
  test   3   0   0 wz--n- <29,99g <29,99g

# lvcreate -L8G -n root test
  Logical volume "root" created.

# lvconvert --type raid5 test/root
  Using default stripesize 64,00 KiB.
  Replaced LV type raid5 (same as raid5_ls) with possible type raid1.
  Repeat this command to convert to raid5 after an interim conversion has finished.
Are you sure you want to convert linear LV test/root to raid1 type? [y/n]: y
  Logical volume test/root successfully converted.

# lvs -a -o name,lv_size,copy_percent,devices,lv_layout
  LV              LSize Cpy%Sync Devices                           Layout    
  root            8.00g 100.00   root_rimage_0(0),root_rimage_1(0) raid,raid1
  [root_rimage_0] 8.00g          /dev/sda(0)                       linear    
  [root_rimage_1] 8.00g          /dev/sdb(1)                       linear    
  [root_rmeta_0]  4.00m          /dev/sda(2048)                    linear    
  [root_rmeta_1]  4.00m          /dev/sdb(0)                       linear    

# lvconvert --type raid5 test/root
  Using default stripesize 64,00 KiB.
  --stripes not allowed for LV test/root when converting from raid1 to raid5.
Are you sure you want to convert raid1 LV test/root to raid5 type? [y/n]: y
  Logical volume test/root successfully converted.

# lvs -a -o name,lv_size,copy_percent,devices,lv_layout
  LV              LSize Cpy%Sync Devices                           Layout             
  root            8.00g 100.00   root_rimage_0(0),root_rimage_1(0) raid,raid5,raid5_ls
  [root_rimage_0] 8.00g          /dev/sda(0)                       linear             
  [root_rimage_1] 8.00g          /dev/sdb(1)                       linear             
  [root_rmeta_0]  4.00m          /dev/sda(2048)                    linear             
  [root_rmeta_1]  4.00m          /dev/sdb(0)                       linear             

# lvextend --version
  LVM version:     2.02.184(2) (2019-03-22)
  Library version: 1.02.156 (2019-03-22)
  Driver version:  4.39.0
  Configuration:   ./configure --prefix=/usr --build=x86_64-pc-linux-gnu 
  --host=x86_64-pc-linux-gnu --mandir=/usr/share/man --infodir=/usr/share/info 
  --datadir=/usr/share --sysconfdir=/etc --localstatedir=/var/lib 
  --disable-dependency-tracking --docdir=/usr/share/doc/lvm2-2.02.184-r5 
  --htmldir=/usr/share/doc/lvm2-2.02.184-r5/html --enable-dmfilemapd 
  --enable-dmeventd --enable-cmdlib --enable-applib --enable-fsadm 
  --enable-lvmetad --enable-lvmpolld --with-mirrors=internal 
  --with-snapshots=internal --with-thin=internal --with-cache=internal 
  --with-thin-check=/sbin/thin_check --with-cache-check=/sbin/cache_check 
  --with-thin-dump=/sbin/thin_dump --with-cache-dump=/sbin/cache_dump 
  --with-thin-repair=/sbin/thin_repair --with-cache-repair=/sbin/cache_repair 
  --with-thin-restore=/sbin/thin_restore 
  --with-cache-restore=/sbin/cache_restore --with-clvmd=none 
  --with-cluster=none --enable-readline --disable-selinux --enable-pkgconfig 
  --with-confdir=/etc --exec-prefix= --sbindir=/sbin --with-staticdir=/sbin 
  --libdir=/lib64 --with-usrlibdir=/usr/lib64 --with-default-dm-run-dir=/run 
  --with-default-run-dir=/run/lvm --with-default-locking-dir=/run/lock/lvm 
  --with-default-pid-dir=/run --enable-udev_rules --enable-udev_sync 
  --with-udevdir=/lib/udev/rules.d --disable-lvmlockd-sanlock 
  --disable-udev-systemd-background-jobs --disable-notify-dbus 
  --with-systemdsystemunitdir=/lib/systemd/system CLDFLAGS=-Wl,-O1 
  -Wl,--as-needed

# lvextend -L+1G test/root
Bus error

# dmesg | tail
[ 6582.162403] md/raid1:mdX: active with 1 out of 2 mirrors
[ 6582.229526] mdX: bitmap file is out of date, doing full recovery
[ 6582.231064] md: recovery of RAID array mdX
[ 6623.103487] md: mdX: recovery done.
[ 6648.042134] md/raid:mdX: device dm-2 operational as raid disk 0
[ 6648.042136] md/raid:mdX: device dm-4 operational as raid disk 1
[ 6648.042439] md/raid:mdX: raid level 5 active with 2 out of 2 devices, algorithm 2
[ 6648.064165] device-mapper: raid: raid456 discard support disabled due to discard_zeroes_data uncertainty.
[ 6648.064167] device-mapper: raid: Set dm-raid.devices_handle_discard_safely=Y to override.
[ 6712.237731] traps: lvextend[13881] trap stack segment ip:556f10ddc428 sp:7ffc4d232010 error:0 in lvm[556f10d44000+106000]


Reverting back to RAID1 solve the problem.

# lvconvert --type raid1 test/root
Are you sure you want to convert raid5 LV test/root to raid1 type? [y/n]: y
  Logical volume test/root successfully converted.

# lvs -a -o name,lv_size,copy_percent,devices,lv_layout
  LV              LSize Cpy%Sync Devices                           Layout    
  root            8.00g 100.00   root_rimage_0(0),root_rimage_1(0) raid,raid1
  [root_rimage_0] 8.00g          /dev/sda(0)                       linear    
  [root_rimage_1] 8.00g          /dev/sdb(1)                       linear    
  [root_rmeta_0]  4.00m          /dev/sda(2048)                    linear    
  [root_rmeta_1]  4.00m          /dev/sdb(0)                       linear    

# lvextend -L+1G test/root
  Extending 2 mirror images.
  Size of logical volume test/root changed from 8.00 GiB (2048 extents) to 9.00 GiB (2304 extents).
  Logical volume test/root successfully resized.

# lvconvert --type raid5 test/root
  Using default stripesize 64.00 KiB.
  --stripes not allowed for LV test/root when converting from raid1 to raid5.
Are you sure you want to convert raid1 LV test/root to raid5 type? [y/n]: y
  Logical volume test/root successfully converted.

Additional info :
Similar problem occurs with fedora 31 on physical computer and LVM 2.03.06(2) (2019-10-23)
Here message is "segmentation fault".