Do I have to worry about transmission problems with my 2018 pathfinder?
That is up to you . Your vehicle does have a warranty which is explained in the owners manual . Most real mechanics recommend a fluid exchange every 30000 miles which your manual does not. That is just good advice . The other thing would be to trade when warranty has expired but your next vehicle could be the same worry and a different problem .
Nissan’s CVT automatic transmissions have a history of problems so yes, I’d be a bit concerned. As @VOLVO-V70 said, you can mitigate potential problems with preventive maintenance, and your warranty is your friend.