4k rsolution issues in Apple tv app for Panasonic z95B fire tv os

The app's interface is in 4K, but once playback starts, the maximum resolution achieved is 1920 FHD. I have no issues with Dolby Vision or audio, but the resolution is a problem. The app is running the latest version (ATV 17.0.0) for the Fire TV AFTGOLDFF, the TV's operating system is on the latest version (8.1.8.0), and my data connection speed is 300 Mbps. I don't know what to do to get series and movies to play correctly. It's a premium TV, and the situation is frustrating. Any help would be appreciated—thanks.