Yeah, I honestly have no idea why that thing happens where you cant jump, because you can jump from a platform sinking like a sofa, but when the lava level changes it does the falling thing (and also with chairs for some reason), but I'll definitely consider adding coyote time after the jam, as it didn't even cross my mind, as I had about an hour before the deadline to playtest. Thank you for the comment!
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.