This wall 'bug', is something that has always been there since the start of TW. I've noticed it very early on w6 already. What happens is simple; you are building walls when you get attacked. Your walls get rammed, but you are still building your walls. However, your village HQ didn't get the update that your walls are already knocked down, and keeps building the same lvl of walls for the same time as it did before, even if the wall lvl you have to build now is cheaper and much faster built. But, if you don't cancel 1 lvl so it updates in the HQ and does start building the correct level, it does usually automatically update itself after the first wall lvl is built at a longer time. So the problem is usually confined to only 1 wall lvl being built slow.
But it is very nice to see that they worked something out so quickly. Makes me wonder, did no-one else ever notice that before, or didn't anyone, like me, just never complain?