I have such a passionate hate for S4B. This also happens to me, and the logging options mentioned below being enabled don't make a reliable difference: the logs don't always save!

Additionally, group chats often split in to two windows where the conversation suddenly continues in a second thread even when no one joined/left... and then you try and reply and it jumps to another conversation on its own.

People in my office have recorded this because it's so obtuse and unbelievable.