I am investigating this issue, but I am not able to reproduce the bug in this scenario. Could you tell me at which difficulty you are playing the scenario? Also, do you have this issue every time you play it?
If you do, please add a recording of the game here.
First time i tried was on legendary difficulty. I encountered the bug when all the enemy units left on the second wave. (I had monks convert their army and i attacked yellow early on)(dunno if this information helps) . Killed off all enemies in their camp and did not beat the scenario. Then i tried on story difficulty and beat the scenario without encountering any bugs. After that I beat the scenario on legendary difficulty without encountering any bugs.