Key frames disappearing when assigning avatar to the .fbx with the animation

Hi there,

I have a animal humanoid character and I've created an avatar to him, everything looks fine here.
We have a .fbx file for each of the animations, I import those animations in Blender and the preview of .fbx the animations looks fine and the same we can see in Blender.

However when I assign the avatar of the character to all the .fbx with the animations, I noticed that some keyframes of that animation disappear (this always happens with some particular bones).

Did that happen to anyone? How can I prevent such data from getting lost?

Thanks in advance.