Google Flights has stale info (and matrix) from time to time. They seem to have gotten better with phantom inventory vs a couple of years ago, but it definitely still happens.

Also, a lot of these premium fares are dual booking inventory fares, so regardless of whether I,R,P, etc. is the primary booking code, if inventory in lower economy booking codes opens up, the premium fare can often drop (even if the booking code remains the same).

Just happened to me today on a client booking...Premium Economy price dropped $50 one-way from time reservation was made to ticketing (overnight). Booking inventory "P" didn't change.