ext4: Funny characters appended to file names

Dear Linux folks,


Using Debian Sid/unstable with 5.9.11 (5.9.0-4-686-pae), it looks like 
the last `sudo grub-update` installed modules with corrupted file names. 
`/boot` is mounted.

$ findmnt /boot
TARGET SOURCE   FSTYPE OPTIONS
/boot  /dev/md0 ext4   rw,relatime
$ ls -l /boot/grub/i386-pc/
insgesamt 2085
-rw-r--r-- 1 root root   8004 13. Aug 23:00 '915resolution.mod-'$'\205\300''u'$'\023\211''鍓]'$'\206\371\377\211\360\350''f'$'\376\377\377\205\300''ur'$'\203\354\004''V'$'\377''t$'$'\030''j'$'\002''胒'
-rw-r--r-- 1 root root  10596 13. Aug 23:00 'acpi.mod-'$'\205\300''u'$'\023\211''鍓]'$'\206\371\377\211\360\350''f'$'\376\377\377\205\300''ur'$'\203\354\004''V'$'\377''t$'$'\030''j'$'\002''胒'
[…]
$ file /boot/grub/i386-pc/zstd.mod-��u^S�鍓\]�����f���ur��^DVt\$^Xj^B胒 
/boot/grub/i386-pc/zstd.mod-��u�鍓]������f�����ur��V�t$j胒: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripped

Checking the file system returned no errors.

     $ sudo umount /boot
     $ sudo fsck.ext4 /dev/md0
     e2fsck 1.45.6 (20-Mar-2020)
     boot: sauber, 331/124928 Dateien, 145680/497856 Blöcke

This causes GRUB fail to load the module, and it falls back into rescue 
mode.

Any idea, what might have happened. It’s a degraded RAID, and I only use 
one drive since several years, but never deactivated it, and `/dev/md0` 
still shows up.

$ more /proc/mdstat
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] 
[raid4] [raid10]
md0 : active raid1 sdb1[0]
       497856 blocks [2/1] [U_]

md1 : active raid1 sdb2[0]
       1953013952 blocks [2/1] [U_]

unused devices: <none>

Kind regards,

Paul