Going through the older 2TB storage drives and comparing some of the data with the current NAS data, it is safe to assume the data corruption event started around mid-March 2023 and ended around early April 2023, months after the September 2022 upgrade. May 2023 is when the bad sectors were reallocated.

Seen this behavior on a suspect hard drive with pending bad sectors in August 2021: on a test Windows XP install using a suspect Seagate 320GB HDD, I copied a large archive to the NAS, then I copied that file to the test system. On the test system, the file was corrupt, even after a few retried copy attempts from the NAS. Of course, on the NAS, it was perfectly fine; hashes matched the one on the laptop but not on the test system. Final attempt was to copy that file to the test system using a USB stick, and that eventually worked.

Of course, without file system integrity checks, this is the result. I'm just usually very good at catching these so far.