The Spelunkery mod adds a fluid block called "Dimensional Tears" to prevent the player from falling into the void (it instead teleports you to your spawn point.) Booflos, not detecting the void beneath them, therefore jump off and into the Dimensional Tears and seemingly get deleted as I have not found any at spawn.

It seems that can be fixed using the portal_fluid_immune entity type tag from Spelunkery and adding Booflos to that