I think it would had been a touching moment for the mechanical hand to fall, but I agree that a Force ghost Luke would have both hands in tact. Ultimately, kind of a non-issue because just as Rian Johnson forgot to film that little detail, I bet many of us that watched it also didn’t think about it, me included.
As with Leila and Chewie ignoring each other after Han’s death isn’t the standard explanation, “because the Force”?