Alethonym projectiles have a relatively high chance of impacting any water on the ground and getting stuck. I don't use or have Mountaintop so I can't say anything for it. A similar but unrelated potential bug which also needs mention is that Alethonym projectiles in Crucible sometimes seem to very very rarely get voided and hit neither Guardians nor the wall behind them (no explosion sound). I don't know if that one would be an interaction with melee hitboxes/latency stuff but I also don't have it recorded.