Here's the file
Every frame of the object player had different size, falling animation frame had height bigger than standing frame. It was constantly switching between those frames because collision mask of the falling frame reacted to the solid, behavior switched to "is on floor" which changed frame to standing, standing frame was few pixels shorter than falling frame, so it lost it's collision with solid, behavior switched to falling again, and so on.
Edit> There are two (maybe more?) ways to get rid of it
- set each animation frame to the same size
- use dummy box with platform behavior and position animations to it.(I used this one)
I erased redundant animations, animation property "auto mirror" takes care of it, you just need to set the sprite's angle.