I've been using this mod for a while now (on v10.4, but about to update to the newest), and I've also seen this (I've only played singleplayer though). Every time I kill a boss (or any enemy) and level up, nothing that would normally occur on that enemy's death occurs. No drops, no hardmode, Dungeon Guardians still spawn, and so on. The most amusing was when I got an early Slime Rain one run, and each time I killed King Slime and leveled up, the count of slimes killed until he spawned would reset, along with the event timer. I had to kill him three or four times before I could avoid leveling and stop the slimes.The second one is a bigger issue. Upon beating a boss (be it single player, multiplayer, and the two version I played) there seems to be about a 50/50 chance that it won't drop any loot and any events that would normally happen as a result don't spawn, but I can still get experience and level up. For example, I killed the Wall of Flesh once, and when I beat it no loot spawned, the blocks that hold the loot didn't spawn, and hardmode didn't activate. The weird part is, when playing on my friend's multiplayer server and the glitch happens, it comes paired with a train of other weird visual glitches, such as the message counting who did what damage not including me (but the damage is split as if I had done damage, minus actually showing my name), the game sort of freezing as if lag is happening for an extended period of time, and the previous bug where loot didn't drop.
I haven't tested it to see if it only ever does this when you kill a boss and level up on it because that's kind of hard to replicate, but it is still weird.
Also, this mod is great, and you and jopojelly apparently got it and TerraCustom to work together while I was busy with real life. They're two great tastes that taste great together!