Tldr workaround: don't equip items on units during combat phase. We'll get to patching it (if we haven't already? I've been off on PTO to recover).

...

Actually I was taking some time off since I've not really had a break since I started. Tried to fix it from home, and diagnosed a little more, but it got into areas of code I'm not familiar/confident about yet. Passed the info up the chain to someone still doing fixes.

As far as I could tell, it wasn't the Client side that was the issue. I got reasonable reproduction rates when dragging an item onto an already equipped unit, while it was in combat, so I think some new server side code may have introduced the bug?

No guarantees on when it will be fixed but I'm sure it will be fixed.