FTP is your power until you reach to lactate threshold. That could be 35 minutes, it could be 90. Over the years it devolved into average FTP for an hour, but its technically not.

Strava, Zwift, Trainerroad, and other companies all have tests to find your FTP but none of it is accurate unless you know your lactate threshold. Finding your lactate threshold isn't easy so people just use an hour as a baseline. Really FTP as a standard should be abandoned for something more concrete.

At the end of the day, FTP is the number we use to set our zones for training. I move mine around all the time without testing. If workouts are too easy, more it up, if the oppo move it down.