Question Mecanim Discarding "Inbetween Humanoid Transforms and Rotation Animation"

I have a humanoid avatar exported from Blender as an FBX file. This avatar was exported from an older game engine, and has some extra bones used in it's animations. All the individual animations in the file are recognized and are able to be imported, but I get the import warning for all of them:

Clip '[Name of the animation clip]' has import animation warnings that might lower retargeting quality:

    '[Name of a bone in the rig]' is inbetween humanoid transforms and has rotation animation that will be discarded.

    ...

...

This continues for multiple bones in the avatar. From what I could gather, (correct me if this is wrong) this is Mecanim trying to convert the animation to something it can retarget to a standard humanoid rig. This breaks all the animations severely for my imported avatar, as now major bones aren't being animated as they should.

The animations are specifically made and match up to my imported avatar, they wont be used on other humanoids. Is there some way I can tell Mecanim to not attempt this conversion? I've tried playing around with the mask, deselecting and selecting both the humanoid mask and bones in the transform mask, none of which changed anything.

I'm using Unity Version 2019.4.31f1.

 

If you want to import your rig as a humanoid, there can't be any extra bones sitting in between any of the human joints. You can have extra bones, but they have to be "leaf bones". For example, if you have an upper arm then a twist bone, then a forearm, the twist bone is in the middle of the humanoid structure and won't work. If instead the twist and the forearm are children of the upper arm then it will work. However, for the extra bones to animate you have to make a mask that includes those bones and apply that mask to the animations you're importing.

If this is an old game then I'm going to assume it wasn't doing anything like remapping animations in real-time. In that case, I'd just import rig as generic. when I'm dealing with advanced skeletons I go generic. If it's a simple character then I go humanoid.