So I just started using electric fences for the first time in A21 and I haven't had any issues. I didn't put in blocks to catch the low-profile zombies like dogs and spiders (though crawlers still get through unscathed).
It's not a bug. From what I understand the change in wire placement was intentional so that it would catch the shorter variety as well, again, except for crawlers. Do we really the fence to stop them though? 😆