Reposting this as my original post got no attention. My Pebble companion pet is grayed out in my pet journal, despite my having verifiably gotten the Rock Lover achievement on 8/25/2024, and memories of showing off my pet to my guild mates after acquiring it.
I originally opened an in-game ticket requesting restoration, to be told by both Game Master Shertoxfai and Game Master Phalastirgue there was nothing they could possibly do about it. Both of them suggested I create a post in the support forums.
The pet isn’t available through the Stable Master, and it feels to me as though the situation could be resolved through restoring the original item that teaches the companion pet after finishing the achievement (https://www.wowhead.com/cata/item=60869/pebble) but I don’t know the internals of the game for why that might not actually be an option.