I had resolved it in my case. It seems caused by mismatch of iOS min support version and Watch OS min support version. For my app, I set iOS support from iOS 13, and watch OS support from 5. But the iOS 13 is match from Watch OS 6. So I set Watch app support from 6.0, the TF builds could install normally.

After I install this app in Mac OS, I got the info.plist in app/Watch/WatchApp/Info.plist, It seems the key [ISIconPlatform] had add to it after processing from AppStore. Left side is original file, right side is from TestFlight.

I got similar issue, I just do some mirror changes in my code, and I could submit it successful before 2 days. ITMS-90338: Non-public API usage - The app references non-public selectors in XXXX: debug:, estimatedProgress, evaluateScript:, imageForKey:, info:, initWithFrame:configuration:, isMainFrame, navigationDelegate, navigationType, newSocketQueueForConnectionFromAddress:onSocket:, onSuccess:, parentFrame, setBundle:, setImage:forKey:, setNavigationDelegate:, setProcessPool:, setSelectionGranularity:, targetFrame, userContentController, warn:, websiteDataStore. If method names in your source code match the private Apple APIs listed above, altering your method names will help prevent this app from being flagged in future submissions. In addition, note that one or more of the above APIs may be located in a static library that was included with your app. If so, they must be removed.

After several test, I found if call the method or property of CLLocationManager, it will show the blue arrow on the task bar. Not matter which property or method.