It definitely affects single-player, but may also affect multiplayer. Having too many monsters on a lot and attempting to fuse something causes the character to be unable to fuse anything, be stuck in 1 direction, and be unable to fight and causes the game to be un-save-able (so, a desynchronization-like glitch). To be fair, the lot where I have this happen has a ton of monsters pre-loaded on it via house building mode, but that doesn't change the fact that I need to clear out 3-4 levels of monsters to ensure that I can fuse safely. (It does, however, make for a great challenge lot.) I can attach a copy of the lot here, provide a save before the game loses the ability to do so (the loading icon appears, but never stops), or attach a log.