I'm assuming you're using CharacterBody2D and is_on_floor() to check for if the player is grounded. In that case when the lava and thus the objects lower the player stays in the air for a frame before falling and getting grounded again. So is_on_floor() returns false. I believe AnimatableBody2D should force the player to stick to the platform as its moving. If you're already doing that then I've no idea.