The file system corruption runs deeper. After a couple of hours, the system bailed with
EXT4-fs error (device md0): ext4_lookup:1856: inode #120979457: comm updatedb.plocat: iget: bad extra_isize 9234 (inode size 256)
Aborting journal on device md0-8.
EXT4-fs error (device md0): ext4_journal_check_start:83: comm rs:main Q:Reg: Detected aborted journal
EXT4-fs error (device md0): ext4_journal_check_start:83: comm systemd-journal: Detected aborted journal
EXT4-fs (md0): Remounting filesystem read-only
which most definitely does not look good :-/