Consider the circuit diagram below-
Two batteries of different voltages are placed in parallel, however as I was told that KVL is applicable to all loops if there is no varying magnetic fields, applying KVL to that loop yields (going in the clockwise direction) -$$-E_2+E_1=0 \\ \implies -6V+12V=0 \\ \implies 12V=6V$$ Which is absurd. I searched for the answer quite a while and found some similar question where an answer explicitly address this condition. But I want to know why I can't apply the KVL here when it is applicable for all loop?