Great question — answering as the UphillTech team:
It turns out that the issue had little to do with mdadm. When I tried e2fsck to see if there were bad blocks in the partition, even though I wouldn't expect to see a valid ext file system, it pointed to the real problem: # e2fsck /dev/sda1 -L /root/badblocks.text e2fsck 1.47.2 (1-Jan-2025) ext2fs_open2: Bad magic number in super-block e2fsck: Superblock invalid, trying backup blocks... e2fsck: Bad magic number in super-block while trying to open /dev/sda1 The superblock could not be read or does not describe a valid ext2/ext3/ext4 filesystem. If the device is valid and it really contains an ext2/ext3/ext4 filesystem (and not swap or ufs or something else), then the superblock is corrupt,...
Full answer by Gary Dale: https://superuser.stackexchange.com/a/1940614 (CC BY-SA 4.0).
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