Opponent stats should be lifetime by default, so that should not be the case.
Is this for all opponents or just some? Are you sure you've seen these players before?
Are you playing cash or tournaments?
There is an option in the Hud to view session only stats either from the PT-icon menu or by double-clicking a player's stats, but you have to enable that manually so I think you'd know if you'd done that.
There are also a few settings in the Hud Options (Hud > Edit Hud Options) which can filter players' stats, but again they are not enabled by default. Did you change anything there?
Next time you see a player that you know you've played before please check their stats in the main database (from the player list select "Choose new player" and type their name) to see if you have data on them.
If so, attach the PokerTracker4.log file from C:\Users\yourname\AppData\Local\PokerTracker 4\ (which you can access from the PT4 File menu) to a
Support Ticket along with a
screenshot of what you see in the Hud.
If you don't have any data on a player you know you've played before please open a
Support Ticket anyway and attach your log and we'll see if there is anything to suggest a problem.