Sometimes, if I toggle-threads in a folder, some of the messages in that folder disappear. If I toggle-threads again, they're back there.
Upon discussion over at IRC, it seems that it might be caused by a missing root node and multiple siblings. We should solve this in a different way (disappearing messages is no good.
Might be related to the first report here: https://todo.sr.ht/~rjarry/aerc/70