UPDATE: I figured out what the problem is and it seems like a bug. Collector for ArcGIS will not take a map area offline if there is a copy of a feature service in the webmap. So if I make a copy of a functional/sync enabled layer in the webmap and save the webmap, I can no longer make an offline area in the new Collector where I could do so just fine before I made the copy of the layer. The same webmap with a copy of a layer works as expected and downloads in Collector Classic. Is this a bug?
Original Post: I've been testing the new Collector to prep for transitioning our organization and have noticed that some web maps will download in Collector Classic (and have always worked fine in Collector Classic), but throw errors when trying to save a map area in new Collector. The error is a generic, "Download Failed" with no other information. My workaround has been to create new maps and that has worked fine for me, but I have users with existing webmaps that are experiencing the same problem and would like to tell them if there's another fix other than make a new map or use Collector Classic.
Are there any reasons why this could be, or things I can troubleshoot? All map layers are sync enabled, webmap has offline mode enabled, there's at least one editable layer in the map, I'm not trying to download an egregiously large or detailed area, etc.
Why would a map download work in Collector Classic and not Collector for ArcGIS? I'm wondering if anyone else out there is experiencing this too.