So, codeshares definitely aren't functioning great in the app. In addition to the
flight status issue that I mentioned upthread, check-in etc was a bit funky.
In the trips list, the blue check-in button does show up at the appropriate time. But once you check in, it doesn't go away like it does on a normal AC-marketed flight.
The home screen will also continue to tell you the flight is open for check-in.
For a while, the BP wasn't showing up in the list of BPs despite app restart/refresh etc. Although for some reason that seems to have fixed itself.
I also had a weird experience with cancelling check-in. I tried to modify the booking through the UA website, and it wouldn't let me. I realized it was probably because I was already checked in, so I used the AC app to cancel check-in. But when I went to check back in, it told me I was checked in and offered me my BP as soon as I hit the blue button, without making me go through any of the steps, as though it had never checked me out. And the BP never went away after cancelling check-in like it usually does. I asked a concierge to check whether my check-in had ever been cancelled, but they were unable to verify (surely DCS should have an audit trail for this, even if not accessible to concierge?). It's possible the error when I initially tried to make the change at UA was unrelated to my being checked in, and that my attempting to cancel check-in had no effect - whatever error I encountered may have resolved itself independently of that. But I did try several times unsuccessfully, on different platforms, to modify the booking with UA, and it worked immediately after I cancelled the check-in, so that suggests to me there's a decent chance that cancelling the check-in did something.
I'm not sure how much of this is the new (2019) app vs Amadeus DCS. I used to frequently fly on UA codeshares of AC flights, mostly to IAH/EWR on the way to LatAm, and I don't ever remember having issues like this. But all of those experiences were 2019 and prior with both the old app and RES III.