bug Invisible Hercules mech from Malta! Screenshots.

[Excalibur]

Not much else to add. I feel like it might have been caused by accidentally killing one of the mechs while it was pathing to merge with another in its spawn, but I can't be certain.

[Rudra]

I've seen this bug like twice. Across both Live and HC. Basically, you damaged one or both Hercules Titans in a single spawn enough that it triggers a merge, like you said. The two Titans get close enough to start the merge, animation starts, but one of the Hercules finishes being destroyed at start of animation. Game goes "Wait a minute, can't summon the Zeus with a single Hercules" and the summon does not happen. Normally, this results in a single visible remaining Hercules that continues the fight. However, if the timing is close enough, both Hercules have started their de-spawn so the game can summon the Zeus. Since the summon is aborted, the partially de-spawned Hercules remains to fight as it is supposed to since the Zeus does not spawn, but it is not rendered because it is also not supposed to be there any more since the summoning animation for the Zeus has already started. Voila, invisible Hercules Titan remains to try to rip your face off. Since both times this happened to me I was on an MM, my pets didn't care it was invisible and still smashed it. I'm not sure this problem can be fixed though, since it is a very specific window of time under very specific circumstances that it happens.