Setting a blank sprite as the mask fixed it (also setting the idle sprite as mask seems to work), i didn't tried it because the sprites have slightly different sizes so i would need a different mask for every sprite, but this are placeholders so it's really a non issue. Can you elaborate a bit why it happens? I'm curious.
Because the jump sprite and idle sprite have different collision sizes (masks) the character is switching between the idle and jump sprite. It lands on the ground so it switches to the idle sprite, but wait, the idle sprite isn't as big as the jump sprite so it switches back to the jump sprite but wait now it is on the ground again cause the jump sprite is bigger so it switches to the idle sprite again. And it goes on like this forever.
Does that makes sense?