iPhone locks when watching a video

This recently started. Whenever I watch a video on my iPhone either through a browser, or a video streaming app, my phone goes into lock mode, even when I have the video in full screen mode. Some of the suggested fixes have said to turn off auto lock feature in settings but that’s not a good solution at all. Am I supposed to turn auto lock on and off anytime i watch a video? Erasing cookies and website data do not work, and restarting phone does not work either. The iOS is up to date to the latest version.