What is happening is not a glitch at all, it is how the game works. Oil pots don't spill if a wall is knocked down, however, they can be destroyed by catapult fire. Here are a few pictures of oil pots in battles I have just recently fought.
http://prntscr.com/lf8pfohttp://prntscr.com/lf8pz3http://prntscr.com/lf8q2zSo in your case, what is happening is the walls are being knocked down but the oil pots are still there (as they are supposed to be) and in later battles, they start "vanishing" because the catapults are destroying the oil pots.
Hope this was helpful, any questions?