Tested with all my kernels, the problem comes from the nouveau driver:
(using dracut version: dracut-046-8.git20180105.fc27.x86_64)

mkdir -p /var/tmp/dracut.bMqSbg/initramfs
DRACUT_FIRMWARE_PATH=/usr/lib/firmware/ /usr/lib/dracut/dracut-install -D /var/tmp/dracut.bMqSbg/initramfs --kerneldir /lib/modules/4.14.11-300.fc27.x86_64/ -m -s drm_crtc_init =drivers/gpu/drm --verbose --debug

=> result:
[...]
dracut-install: hash hit items for '/lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/ttm/ttm.ko.xz'
dracut-install: dracut_install '/lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/ttm/ttm.ko.xz' '/lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/ttm/ttm.ko.xz' OK
dracut-install: Ignoring /lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/cirrus
dracut-install: Ignoring /lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/nouveau
dracut-install: Handling /lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/nouveau/nouveau.ko.xz
dracut-install: ERROR: installing module '�, eau'

If I add the -o option, I don't have the error. I get the following output:
[...]
dracut-install: hash hit items for '/lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/ttm/ttm.ko.xz'
dracut-install: dracut_install '/lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/ttm/ttm.ko.xz' '/lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/ttm/ttm.ko.xz' OK
dracut-install: Ignoring /lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/cirrus
dracut-install: Ignoring /lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/nouveau
dracut-install: Handling /lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/nouveau/nouveau.ko.xz
dracut-install: Ignoring /lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/nouveau
dracut-install: Ignoring /lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/vgem
dracut-install: Handling /lib/modules/4.14.11-300.fc27.x86_64//kernel/drivers/gpu/drm/vgem/vgem.ko.xz

ls -l $(modinfo nouveau |grep firm|awk '{print "/usr/lib/firmware/"$2}')
gives no missing firmware files. So I assume that there is another reason why the module is ignored.

Possible fix for the garbled memory:

https://github.com/dracutdevs/dracut/commit/6c7776e34456b420344fee2cf279ae30413058e1

In regard to the "ignore", what is the output of:

$ grep . -r /lib/dracut/dracut.conf.d /etc/dracut.conf.d/

and especially

$ grep nouveau -r /lib/dracut/dracut.conf.d /etc/dracut.conf.d/

$ grep . -r /lib/dracut/dracut.conf.d /etc/dracut.conf.d/
/lib/dracut/dracut.conf.d/10-plymouth-theme-breeze.conf:# This is a hack that is currently necessary for package: plymouth-theme-breeze
/lib/dracut/dracut.conf.d/10-plymouth-theme-breeze.conf:# due to the fact that `dracut` only includes the top-level directory of theme folders
/lib/dracut/dracut.conf.d/10-plymouth-theme-breeze.conf:# By default. See also https://bugs.kde.org/show_bug.cgi?id=371276
/lib/dracut/dracut.conf.d/10-plymouth-theme-breeze.conf:if [[ "$(plymouth-set-default-theme)" =~ ^breeze ]]; then
/lib/dracut/dracut.conf.d/10-plymouth-theme-breeze.conf:        readarray -t plymouth_theme_breeze_files < <( find /usr/share/plymouth/themes/breeze -mindepth 1 -type f )
/lib/dracut/dracut.conf.d/10-plymouth-theme-breeze.conf:        install_optional_items+=" ${plymouth_theme_breeze_files[@]} "
/lib/dracut/dracut.conf.d/10-plymouth-theme-breeze.conf:fi
/lib/dracut/dracut.conf.d/01-dist.conf:# dracut config file customized for RedHat/Fedora.
/lib/dracut/dracut.conf.d/01-dist.conf:# i18n
/lib/dracut/dracut.conf.d/01-dist.conf:i18n_vars="/etc/sysconfig/keyboard:KEYTABLE-KEYMAP /etc/sysconfig/i18n:SYSFONT-FONT,FONTACM-FONT_MAP,FONT_UNIMAP"
/lib/dracut/dracut.conf.d/01-dist.conf:i18n_default_font="eurlatgr"
/lib/dracut/dracut.conf.d/01-dist.conf:i18n_install_all="yes"
/lib/dracut/dracut.conf.d/01-dist.conf:stdloglvl=3
/lib/dracut/dracut.conf.d/01-dist.conf:sysloglvl=5
/lib/dracut/dracut.conf.d/01-dist.conf:install_optional_items+=" vi /etc/virc ps grep cat rm "
/lib/dracut/dracut.conf.d/01-dist.conf:prefix="/"
/lib/dracut/dracut.conf.d/01-dist.conf:systemdutildir=/usr/lib/systemd
/lib/dracut/dracut.conf.d/01-dist.conf:systemdsystemunitdir=/usr/lib/systemd/system
/lib/dracut/dracut.conf.d/01-dist.conf:systemdsystemconfdir=/etc/systemd/system
/lib/dracut/dracut.conf.d/01-dist.conf:udevdir=/usr/lib/udev
/lib/dracut/dracut.conf.d/01-dist.conf:hostonly="yes"
/lib/dracut/dracut.conf.d/01-dist.conf:hostonly_cmdline="no"
/lib/dracut/dracut.conf.d/01-dist.conf:early_microcode="yes"
/lib/dracut/dracut.conf.d/01-dist.conf:reproducible="yes"
/lib/dracut/dracut.conf.d/02-rescue.conf:dracut_rescue_image="yes"
/lib/dracut/dracut.conf.d/50-nss-softokn.conf:# turn on nss-softokn module
/lib/dracut/dracut.conf.d/50-nss-softokn.conf:add_dracutmodules+=" nss-softokn "
$ grep nouveau -r /lib/dracut/dracut.conf.d /etc/dracut.conf.d/
$

# grep -r nouveau /etc/mod*
/etc/modprobe.d/blacklist.conf:blacklist nouveau
/etc/modprobe.d/blacklist.conf:blacklist lbm-nouveau
/etc/modprobe.d/blacklist.conf:options nouveau modeset=0
/etc/modprobe.d/blacklist.conf:alias nouveau off
/etc/modprobe.d/blacklist.conf:alias lbm-nouveau off

That may explain why the module is ignored, though I did post the bug report because of corrupted name in 1st place.

Although, why would /etc/modprobe.d/blacklist.conf be taken into account when generating an initramfs when --no-hostonly is specified?

(In reply to Harald Hoyer from comment #2)
> Possible fix for the garbled memory:
> 
> https://github.com/dracutdevs/dracut/commit/
> 6c7776e34456b420344fee2cf279ae30413058e1

That makes sense, any test rpm I can try to confirm resolution?

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