There's a crash sort of thing that happens when a would be dead demi summoner spawns another one that happens to be perfect to fuse with. After they fuse the run just stops, and quitting the round is the only way to do anything again
thanks for the update! that should be fixed on the experimental branch (https://newbeings.itch.io/order-automatica-beta)
and will be included in the next update here : )