Lava decay fails to schedule block update

I think this is an old issue that was never revisited but it has started causing issues with my obsidian farm.

when you remove the top layer of source blocks from a lava pool the adjacent source blocks will back-fill the pool. after all the source blocks are removed this residue lava remains indefinitely and does not drain to the pools new level making it difficult to get to the source blocks underneath. also some lava pools will remain for days and weeks with no source blocks at all. the draining mechanics for lava seem to need revisiting.