ok so here is what happened.

Ostland got murdered by Nurgle and i marched to get the city back(which i did), after capturing it back i end my turn. when the new turn starts i get a message that the citizens of Ostland want their homeland back. so i give it back to them(in the message it said something like leave the city) and so i left the city and ended my turn. now this turn(3rd one after the reconquest) i see Ostland popup right beside my city(their Ex-Capital) and proceed to take it over, when the phase was done i get my usual messages for my previous turn and apparently Ostland has betrayed me and the alliance made by Ostland to everybody has been cancelled and they are also at war with me.

the whole phase of giving back their homeland was pretty cool. i hope this issue is easily fixed.