My immediate thought was that there’s some inconsistency with various types of metadata. For example most software will pull the date from the Exif DateTimeOriginal field. But there’s also XMP tags that have the same purpose. Or similar purpose. These standards have plenty of date fields for various uses, and while they serve a noble purpose, the software just craps all over them. (Don’t ask which software. All of them.)

My guess is that at some point of time, one of those tags got updated, but not the other tags of similar purpose. So the program you’re using could be pulling the date from one field, and when you update it, you’re actually changing some other field.

Of course all of this is wild because usually no one needs to touch the datestamp anyway (unless you, like, have to correct daylight saving time or clock drift or something). Software changing this to a batch import time? That’s weird and silly.