Lava doesn't go away when I remove the source block. After some experimenting, it appears that the minecraft:lava block didn't go away after I used a bucket, it just became invisible. Textures also behaved as if it wasn't there. (generated minecraft:flowing_lava looks like it has no source, or that it is coming from other blocks if the same type) The only way to remove it was to place a block where the source block should be. Did not check to see if the bucked used be used again. Check  this video of the issue. (Doesn't include the "experimenting" or the solution I stumbled upon. Edit: after removing the source block, the lava in the bucket ceased to exist.