I am running into somewhat similar issue as well. iPhone XS Max, iOS 14 Beta 4 paired to Apple Watch 4 with v6.2.8. Can't update to Watch v7 beta, not a developer. Been waiting on the public 7 beta....
In the Watch app on the XS, the only complications that can be chosen for a watch face are the Apple ones (and it's missing the date complication, weird). The other apps I have installed on the watch show up under the 'Complications' option (Dark Sky, AutoSleep, Autowake and 3 others), but I can't choose them as a complication in a watch face.
The apps do work if selected though when pressing the crown.
Any thoughts? Reset/re-pair the watch (haven't tried).