I agree that it's quite unusual for PSG to finish second. Since few other clubs in the league can match their resources and talent, they are guaranteed to win. But remember that PSG's new manager, Luis Enrique, appears to be shifting the team's emphasis. Acquiring famous athletes is no longer the only goal. Recruiting players who are a good match for the system is becoming more important, as this could affect how they play. But if you change your tactics or bring in new team members, you can run into trouble.
Lens has been performing well as of late, but the number of available players will likely be more important to their long-term success than any other factor. Without greater support, their endeavor may fall short of its goal. The overall direction that Enrique takes PSG in, and whether or not any other team can match Lille's success, will be fascinating to observe
It has already happened, Messi wasn't there yet but Neymar and Mbappe was there and they lost to Lille, they didn't win the league that season, and didn't win the UCL title neither that season, so that season they basically just lost. Not that it will happen again this season, but it is not impossible neither and there is always a potential. Look at last year and how close it was, there was a possibility of it happening again, just one game and things suddenly change.
All in all I am expecting them to win it again anyway, not really seeing them lose it, but at the very least I can say that it will not be guaranteed for them to win, it will be likely for them to win and unlikely for anyone else to win, but not impossible.