I'm facing the same issue. I have 2 Apple Watches, Series 3 Cellular $ Series 7 Cellular. On the Series 7 (latest non beta 9xx) TestFlight 3.3.0 installs apps on to the watch. Series 3 (latest non beta 8xx) will NOT install watch apps.

I’m guessing that it’s going to be a while before apple fixes this issue. does anyone know how to rollback the TestFlight application to an older version?