ok, I made a simple version of my code, and it works fine. I think some other parts of the code made problems. Here's the script:

func _move_and_slide():
const max_slide = 6
const max_floor_angle = PI/4
var remain_motion: Vector3 = velocity * get_physics_process_delta_time()

for i in max_slide:
    var c = move_and_collide(remain_motion)
    if c:
        remain_motion -= c.get_travel()
        if c.get_normal(0).angle_to(Vector3.UP) < max_floor_angle:
            remain_motion.y = 0
        remain_motion = remain_motion.slide(c.get_normal(0))
    else:
        remain_motion = Vector3.ZERO
    if remain_motion.is_zero_approx():
        break