My province suddenly does under my allies control

hi, I have been playing this game for a few months now.

in game 4111905 ,a arms race event map,one of my urban provinces was switched to my allies control.I have just done a decisive battle that wiped out a significant portion of my enemy’s tank force.There was a railroad gun bombarding me,and morale dropped to 1%.And after a few hours (I believe it’s before day change) the province suddenly turned to my allies control,he has not sent any troops in the province,and I have core territory on that urban province.we have not tried to replicate the bug but it has dropped my food production to 0.

And yes,I have taken care of the railroad gun after my troops defeated the tank force

Is this an intentional feature? If yes,would it not be reasonable for the ally to hand back control to you?

i cannot unfortunately upload a screenshot because the file is too big