Apparently over the free weekend, some unknown user decided that it would be fun to set all of Solitaire Expeditions' global statistics to their maximum values.
I contacted Steam and they say that they are unable to do anything about it.
It looks like I am going to have to create an entirely new set of global stats, which is a significant amount of work and means everything will start at zero again.
Personal in-game win/loss records will not be affected; this is strictly related to the global statistics.