I noticed this too, but if you leave combat the auto-feed will occur.

I think it is a) either a bug that the auto-feed won't happen during combat b) the pet button disappears so you can't even use it as a short cut to manually feed, during combat

either way this is a big pain when you are doing chain pulls with a group in W4 and your pet button has disappeared and you have to navigate through avatar>stable>feed