When a block is above a path block, it is automatically supposed to turn to dirt. However, in versions 1.13-pre4 or pre-3 (as far as I have noticed), the path blocks will initially turn to dirt, but if the area below the paths is filled in with paths with /fill, it will not update, rather remain as path blocks. I compared this to 1.12.2, the latest release, and no matter how many time the area is refilled as path, only the top becomes path block (as it should).

As you can see in the later three screenshots, as I continue to put a layer of path below the original layer, the path does not update and turn to dirt, which is surely problematic.