I once have found a solution to this, it’s in this thread at the bottom.
I’ve used Mixamo chars and animations, but this solution also worked on a custom-made character. I’m still yet to test and work on some workarounds for assets from Unity Asset Store and other sources with and without pre-made animations. However, what I understand is that the best solution is using a 3rd party software like Blender to work on the animations outside of Unity. I think that’s how things work. On the other hand, there are multiple workarounds that we can try to get around this issue.
So from what I gathered so far
Workaround 1: Upload the character to Mixamo, download the T-Pose animation with the skin, use that avatar on your character, and animate.
Workaround 2: Switch between Generic/Humanoid rigs. Record animation while in a Generic rig then use that animation in a Humanoid rig. Additionally assuming supporting the avatar mask and IK too.
Workaround 3: Create your animations in 3rd Party Software such as Blender, Maya or others.
Please let me know if you know of any other workarounds or solutions.
Can Unity employees also shed light on this one, why can’t work smoothly with bones using animation rigging in humanoid rigs or are we getting the wrong idea about the humanoid rig? A clarification would be appreciated.
Moreover, I also found this video which I believe would help many people if you haven’t found it already:
How to work with humanoid animations in Unity: