I can mitigate the issue by making sure to change regions then changing back before pulling the lever again.

This happens too if the matchmaker fails to setup a match, restarts the search and finds another one. Bell rings and the game just closes

Other effects observed though not sure is related:

Matchmaker bell rings, but there is no fadeout and I'm stuck in the gym unable to move.

After a failed matchmaking setup, the gym reloads but my feet are stuck to the ground and I can't jump.

Gym and the arena are overlayed