I am 99% sure I know what's happening here having experienced the exact same issue. I too migrated to the single app target and have regretted it for this problem ever since.
My understanding is that in the WatchOS system there are caches for the complications you run on your watch face. These caches include bundle identifiers for the complications, bundle identifiers which change when you migrate to the new single target! So this means the complications on your existing watch face point to what seems like a non-existent app hence the "--". I think the reason this so blatant problem has got through and continued is because Apple expects developers to also adopt WidgetKit for complications instead of Clockkit, and this has a migrator (although I've read plenty of issues with that). The problem is that some developers, including me, are not yet adopting WidgetKit for complications for many reasons including supporting < WatchOS 9, issues with WidigetKit update performance not being as reliable as ClockKit etc....
In theory, with my answer above you would think that just deleting the complications off the watch face and adding them back will resolve the issue. It can do, but often the caches don't clear and you have to either reboot the watch, delete the watch face entirely and start again or various other combinations of fixes. I missed and therefore shipped the problem so I have had to create an entire support document for my users to resolve this bug. I also will not ship an update undoing the problem because then I'll have even more users with watch faces on a variety of complication bundle identifier caches if that makes sense....
So, if you have shipped it, that's a problem. The good news is it won't affect new users of your app, but old users with existing complications will have a pain migrating over.
If you haven't shipped yet, my only advice is don't and revert back to a single target until Apple fixes the issue. (FB 11989396) If you somehow felt like you had to ship a single target you could try changing all the identifiers in your CLKComplicationDescriptors you provide to the getComplicationDescriptors() function. I've not tested this but think it would solve the problem. That said, any existing users with watch face complications will still have to change them for the new ones, but the caching/restart issues should be prevented.
I really hope this is the answer you're looking for and it's helpful, please let me know either way! As I'm sure you can tell from the length of my response, it's given me a great deal of pain in the last 8 months!
