If you disable RunningNoPhysics the above issue doesn’t occur anymore. @EchoReaper also mentioned that if you give the character a Torso, the issue doesn’t occur. This seems to be a problem with Humanoids relying on specifically named parts to properly simulate physics.
Yeah, one of the incubators this spring spent hours debugging their game trying to figure out why this was happening. It is non-obvious and makes it substantially harder to make characters that aren’t human-like unless you know what you’re dealing with.
Every occurrence where humanoid parts are mentioned by name in the engine should be filed as a bug. This is a huge antipattern.
Humanoids suck to work with if you’re trying to make something more complicated. I’ve spent some time this week just making my own custom humanoid with physics - at least the bugs I find with it will be solvable.