Funny, because I also have an issue with the A321 progressive boarding, but mine is completely different:
MSFS2020, GSXpro v 3.5.8
Using the Fenix A321 IAE, progressive automated fuelling/catering, then loading/boarding. The fuelling commences normally and everything is fine. Also, when fuelling is finished, the cargo loaders operate normally. BUT: The PAX don't board. After fuelling has finished, it takes 15 minutes (!) before PAX boarding starts. That's 10 minutes after even the cargo loading finished (...which should normally be parallel to PAX boarding, right?)
After I had this at multiple airports, I have clocked the process:
0:00 Start Loading per Fenix EFB (refuelling and boarding, real time)
0:08 Refuelling (and catering) finished, cargo loaders drive up, loading commences. Note: No PAX boarding!
0:13 Cargo loading finished. Note: A) Still no PAX boarding, no FAs in the galley; B) Cargo doors not closing automatically > done manually
0:23 PAX boarding process starts (Sidenote: Without anything done on my side to trigger it. Just sitting idle for 10 minutes after manually closing the cargo doors)
0:33 PAX boarding finished.
I had not flown the Fenix in a long time, but I have never noticed this behaviour before...