The rare was supposedly hotfixed to allow more players to tag it a few weeks back, but cross-faction tagging is still a problem. I have been on the wrong faction for the tag every time I encounter the rare and it is greyed out. I’m not sure if Chef Chum Platter is still this way as I usually tag it pretty early and have not had an issue getting loot from him.
Edit: it’s possible the tag issue is still number of players, as this morning I am seeing both factions attacking it and it’s grey for me.