Not sure if this insight adds any value to you but I think that there may be something going on with teams and end round conditions that is implied by this solution.
If you have last man standing in order to handle join in progress you have to set it to spectator. I did some tests before and the way that spectator spawn on join works is the players get spawned and instantly eliminated.
I am wondering if your join in progress setting may have anything to do with it. (you really never know when you let epic handle this kind of stuff, and is precisely why I ended up just doing a full replacement, too many inferred dependencies and workarounds involved)
(no idea if this will be useful but just figured I’d check since you seem to be out of options)