Chris,
Excellent troubleshooting, thank you.
The timeline does match up with the automatic purging when placing a detail, that was implemented with version 17.09, and you guys installed on February 22.
Luckily here, everything Land F/X is doing is clearly visible at the command line, so that should give us our best hints.
Up to 17.09, the system would attempt to purge out the detail block before inserting it. After 17.09, it also attempts to purge any blocks within the detail. You can see this at the command line, as it fires the Purge command (you might need CMDECHO set to 1). After that, it fires an INSERT command, also at the command line.
That is the only thing that changed near that timeline.
However, that is only when *placing* details from the Manager. Meanwhile, over in the Update Details function, it uses an entirely different method of inserting the updated details that is not visible at the command line, and it does not attempt to purge anything. The only clue at the command line as it fires is the duplicate block warnings (this can be compared to the similar output when Placing). Yet this code has not been modified since May 2017.
There are several Autodesk forum threads regarding this, for instance this one:
https://forums.autodesk.com/t5/net/dynamic-block-with-associative-hatch/td-p/10023207
Which would support this behavior - that when placing the dynamic block with a command call, like Place Detail does, it works fine. But using an internal insert method, the hatch association is lost.
But where that doesn't line up, is as you mentioned, this was all working fine in 2021 for a period of a few months.
You mentioned your 2021 has not been updated. Yet there is a 2021.1.1 update that was released on April 26. I don't see anything in the release notes that look promising, but it would certainly be worth a shot.
--J