v7.30 was just released with many fixes (announcement). It usually takes several days for Amazon to publish a new update.
Depending on your or your in-laws comfort level, you could sideload the app to their FireStick. Use the FireOS Devices version.
Before sideloading, check the Max H.264 level in the FireStick Plex app settings. Some versions had a bug and set it to 1.0 (fixed in 7.30). It should be 5.1 or 5.2. Check & change if needed.
Also in the Plex app, try disabling the advanced/new video player if you see the option (settings -> advanced). It has caused issues for some users. Using the new player may be Plex Pass only right now. Not sure if you’ll see the option to disable.
