something similar happened with my old gravestone in an older world.
every so often the grave would respawn as if i never claimed my items back. sorta how the bonemass in your world returns as if never slain.
have to imagine just minor bug in saving details/data
if world is old (started before one of the updates) its prone to minor bugs and such

This replay is within the last month. It's solo, so there shouldn't be anyone else jumping in to activate things. Blogmutt disappeared again when I went inside a crypt and came back out.

The recycled Bonemass...?

Sorry, biology teacher here. Should be "Bonemass".

Originally posted by jjrobinson2:

Sorry, biology teacher here. Should be "Bonemass".

No, it's going to be Biomass now and forever.

could be indeed a steamcloud revert issue or someone summons him and did not managed to defeat "biomass" (a.k.a bonemass / big blob / green-mean-jelly-boi)

..why do i want to place down a biomass burner now?..

that or world revert and he happened to be alive in the copy that was saved. Could have happened long ago, too.

Went into a crypt, and he was gone when I came out. His status bar had appeared, and disappeared while I was inside.

The most obvious explanation would be that someone else summoned him... Figure out who has access to your world and start there.... Pretty sure that's the only way a boss could just suddenly appear.