Jungle Valley eldritch altars unable to affect boss

I've run a few dozen Jungle Valleys with Eater of Worlds influence, and have yet to see (to my knowledge) an altar that has a boss option. They only have player or minion options.
Were I to guess, it has something to do with the drop-down spider boss not being "present" on the map when the altar spawns. I have not investigated other maps with similarly-functioning bosses.