I've started having this issue in the last 2 weeks as well. Twice in same scenario, briefly stepped out of the car to drop off my son to school, fob is in my pocket, doors are open. I seat back in and car is off and says key not inside although it's in my pocket. I had to unlock/start the car with the phone app.
Purchased a new fob battery despite not seeing any battery warning, now reading about battery swap quirks to make sure my fob doesn't turn into a brick.
My guess is it's related to a recent update. Running 2020.48.10, started with prior version.