QM FFA: monarch disappeared from barbican after retreat

Please take a lookat this game:
#121337687

At about minute 19:30 the player 灰风 retreated and the monarch was garrisoned in the barbican landmark. When it went down, the monarch disappeared and I was not able to snipe him. No one else did, though. This does not seem to be intendet.

Regards,

puk