Unity Animation: Game Object transform 'restart' when I select another Game Object

Hello! I have a problem when I try to create an original new animation for a mixamo imported model. As one video is better than 10.000 words, here I recorded one:

In min 0:17 I had my first problem, the entire prefab transform changed when I start to record the clip.
In min 0:37 I fixed the rotation of the left leg but when I choose the right one the former is restarted.

The model's avatar is humanoid and is perfectly configured by default. The "Apply Root Motion" is unchecked. And I don't now what other info do you need to help me understand this problem but you can ask me for more.

Thank you all!