Their tamed treasure slime can carry an item inside it to change its colour. If you use the Etheric Gem on it while it has, say, a diamond inside, and then remove the diamond, and then kill the slime and let it respawn, it respawns with a second diamond in it rather than the item it had when it died. If it's not 'properly' fixable, I wanted to suggest either always respawning treasure slimes empty, or including that mob on the mod's blacklist by default.

this should be fixed in 1.0.5, could you test again?

Unfortunately discovered a new bug while trying to test, reported it as its own issue.

Seems to work now! Treasure slime respawns with the item it had in it when it died.