Every weekend for the next month or so, there seem to be no 1st class Advance fares from Euston to Piccadilly. Maybe buying a standard advance for £36 + £15 upgrade it'll be slightly cheaper than the £56 1st off-peak.
This does not just seem to be confined to first class advances. I tried booking a number of journeys last week on the Virgin site wanting to make use of the seat selector. It would only offer me off peak returns / singles, despite advances for the same trains being available on other sites.