I returned to my base after leaving the area and 20+ allays which I had in an XP farm have mysteriously vanished. I checked the whole area, and they did not escape, they are simply gone. Other entities in close proximity were unaffected and were still there, just the allays were missing.

I do have mods, but all of them are client-side. There are no datapacks involved.

All affected allays were holding iron blocks, and had at some point picked up and dropped them. I had unloaded the chunks before, and they had been perfectly fine until now. I have completely ruled out any possibility of them dying.