As several other posts have indicated, I dont think the monorail stops before or during any of the parties (P&P, MVMC, MNSSH, even Night of Joy). If the monorail wasnt running for some period of time on a given night, it seems more likely it was a temporary mechanical or operational issue. Afaik, the only private event during which Disney wouldn't run transportation to and from the Magic Kingdom would be Grad Night.
It makes NO sense to stop it at 5:30 or whatever, given that the Magic Kingdom is open to the entire Magic Your Way pass-holding public until 7 PM.