There is actually a much simpler repro case:
Format volume, create large file, fsck volume, boink:
farblos@sappc2:~$ sudo mkfs.vfat -n MCBACKUP /dev/sdc1
mkfs.fat 4.2 (2021-01-31)
farblos@sappc2:~$ sudo mount /dev/sdc1 /mnt
farblos@sappc2:~$ sudo truncate -s 4294967295 /mnt/bigfile
farblos@sappc2:~$ sudo umount /mnt
farblos@sappc2:~$ sudo fsck.vfat /dev/sdc1
fsck.fat 4.2 (2021-01-31)
/bigfile
File size is 4294967295 bytes, cluster chain length is 0 bytes.
Truncating file to 0 bytes.
*** Filesystem was changed ***
The changes have not yet been written, you can still choose to leave the
filesystem unmodified:
1) Write changes
2) Leave filesystem unchanged
[12?q]? 2
/dev/sdc1: 2 files, 1048577/1961982 clusters
