You're not the only one. It's a widespread issue with TestFlight and Watch apps and it started as of iOS 13.0/watchOS 6.0. Unfortunately, Apple still hasn't fixed it.
As you've discovered, the fix is to toggle the Watch app in TestFlight to uninstall/reinstall. (when I tried to delete one directly from the Watch I ended up with a broken icon and the app not actually being removed)
This year's iOS/watchOS releases have been rough 😬