I am so sorry for this !
You ran into the most terrible bug in the game (I call it the gremlin).
This bug is my phobia every time I see a new let's play of the game on youtube.
I discovered it a few days ago
It is very rare and seems to be provoked under certain specific playing conditions. I just have to study which ones during this week.
Please. Could you tell me what type of computer you use and on which browser you play? Maybe this can help me solve the problem even faster.