Thank you for the answer!
All of the sprites are in a KinematicBody2D node, but I don't animate this body, only its child sprites (body parts).
I have created a simplified project, that contains only one leg.
The node hierarchy is similar to this: cutout animation tutorial
The AnimationPlayer calculates wrong values between the keyframes, or I misunderstood something about how it works.
Here you can see, that I don't change the KinematicBody2D's properties:
I can add new keyframes (as you said), and it seems good in the editor, but the problem still exists between these new keyframes when I play the project.
I hope, I don't need to put keyframes to every 1/60 seconds :)