I'm not sure about "undisputed #1 PFP," but there have been plenty of times where a #1 PFP candidate was dominated.

Toney was considered by many to be the #2 PFP guy (behind Pernell) when Roy dominated him.
Chavez was considered #1 PFP when Pernell (who was considered #2 or tied for 1st) dominated him.

Mosley was considered top 2 (behind only Jones) when Forrest dominated him.