I don't know if anyone else has noticed this but as of late my pet seems to be taking full aoe damage from a large variety of different spells. Avoidance is still listed as an ability on my pet so I'm guessing its a technical error.
The next time you notice this happening, can you list the spell/creature you were fighting?
this issue has been posted on the bug forums alot recently. is effecting lock and dk pets as well plus other pet aspects are broke. blizz just refuses to answer anyone about the issue. i suggest everyone send in an ingame bug report about the issue i know people say flooding them just backs em up but if they refuse to answer if the issue is a known issue or something like they have been with vanity issues and transmog which isnt important, then as a paying customer we should be expected to flood them with issues if we are not satisfied with their services