They will need to fix that. As others have said, just laziness by the painter (shakes head).
All skirtings should all be painted the same way (unless you specified otherwise)
It can probably go on your 3 month maintenance list if you are in a hurry to move in, but better if your SS has the painters coming back for anything else before handover, get them to do it before you move in. (that way you won't need to clear the robe out)