It seems that the Waterlogged biome will spawn itself on top of the Crab King spawner and delete it.
Relevant thread here:
Re-gen worlds till you get a world where CK has been eaten by the Waterlogged biome.
I guess c_announce(c_countprefabs("crabking_spawner")) would suffice when generating worlds, or whatever the debug name for it is.
EDIT: WORLD SEED HERE 1622497803