Join Date: May 2004
Location: SIN (LEJ once a year)
Programs: SQ, LH, BA, IHG Diamond AMB, HH Gold, SLH Indulged, Accor Gold, Hyatt Discoverist
Posts: 8,840
The phone guys probably think there is availability using points for your shortened stay period without validating.
The hotel should be able to modify (shorten) your points booking without availability for points nights, whether they'll do it is a different story, but it doesn't hurt to ask nicely.
Central reservations cannot do the same.