Armies retreating from Besieged cities now?

**Edit**Did some testing and took screenshots

Grombrindal has no movement points

Grombrindal is definitely under siege.

Grombrindal ran away over land and the city is no longer under siege

Was this something introduced in the latest patch or is this known bug? Started playing a VH Tyrion campaign for the achievement in patch 5.3.1 and have noticed occasionally armies will retreat from besieged cities now. Have noticed it once or twice throughout this campaign but most recently had an egregious instance that made me go looking for an answer. Grombrindal declared war on me and took Naggarond, so at the end of his turn he is stuck in that city. Have 2 armies close by so attack with one to besiege the city and then encircle and bring up my second army, happens to be Tyrion’s army in force march. Attack the city again with the first army to initiate the battle and Grombrindal runs away as if he isn’t under siege….what the actual fuck…. Any one else encounter this or something similar????