Joe2019
Correct and full reinstall of an app on RokuOS requires all the steps in order (you must RESTART each device AFTER unstalling the Apple TV app but BEFORE reinstalling the AppleTV app otherwise the app's data/cache is left behind - you must do this for all devices):
- Remove Apple TV app
- Restart device
- Reinstall + reuathenticate Apple TV app
You really should state your model #s, not names: Roku (re)uses the same exact model name for different model #s over time: Settings/System/About
The device with OS15 was 'randomly selected' to receive/test an initial beta pre-Release build of OS15 - the other devices were not (this is how Roku does it for every major OS version upgrade).
You'll be able to upgrade all of your devices once OS 15 is released (sometimes with the same latest 'pre-Release' Beta build already on some devices if Roku decides its stable/bug-free enough).
Again: OS 15 hasn't even been announced, yet alone released, though pre-release builds are being tested on some devices. If you want to wait for OS 15 with the assumption that it will address/fix AppleTV playback for all of your devices, you can of course do that.
Otherwise...
Apple TV content plays without issue on a 3910 (Express+), 3960 (Express), and 3940 (Express 4K), all with OS 14.6.4 Build 9914 (and Apple TV app version 16.0.51), so OS 14.6 itself is not preclusive.
Its likely something in your setup (network config, HTS config, or RokuOS config) is preventing playback with the OS 14.6 devices: Is there an error message? Does it go to a 'black screen' instead of streaming? Is there audio but no video? etc.
Its common that folks with ad-blocking enabled modems/routers/gateways have all sorts of weird streaming issues breaking some/one app/content playback, but not others, etc, so its best to @ least temporarily disable such ad-blocking functionality, along with (on your Roku devices) setting Auto-adjust display refresh rate (framerate matching) = Off, manually configuring Display Type as 4K DV/4K HDR/4K/1080p/720p TV and Digital output format as Stereo, and directly connecting the Roku to the TV/display instead of an AVR, to test the source of the issue.