This is similar to https://bugzilla.redhat.com/show_bug.cgi?id=1993251 , but for blivet-gui. When python-blivet was fixed in response to that bug report, the regular custom install software RAID tests (that use anaconda's own custom UI) started working, but the blivet custom install software RAID tests (that use blivet-gui within anaconda) still fail with the same error: https://openqa.fedoraproject.org/tests/952622#step/disk_custom_blivet_software_raid/24 "mdadm: specifying chunk size is forbidden for this level" so I think blivet-gui is directly specifying the chunk size for RAID sets, and needs also to be tweaked not to do it for RAID1. Proposing as a Beta blocker as a violation of "When using both the installer-native and the blivet-gui-based custom partitioning flow, the installer must be able to: ... Create mount points backed by ext4 partitions, LVM volumes or btrfs volumes, or software RAID arrays at RAID levels 0, 1 and 5 containing ext4 partitions" - https://fedoraproject.org/wiki/Fedora_35_Beta_Release_Criteria#Custom_partitioning
This is not fixed in F35, because F35 is frozen. The fix will need to be pushed through the freeze before this can be closed, assuming it is accepted as a blocker or FE.
FEDORA-2021-d169e03b3f has been submitted as an update to Fedora 35. https://bodhi.fedoraproject.org/updates/FEDORA-2021-d169e03b3f
+3 in https://pagure.io/fedora-qa/blocker-review/issue/398 , marking accepted.
FEDORA-2021-d169e03b3f has been pushed to the Fedora 35 stable repository. If problem still persists, please make note of it in this bug report.