Okay, after troubleshooting for some time, I've found the solution and cause of the problem. I had several instances (copies) of the same hosted feature layer within the Map Viewer, and although they were not active (turned off), they were the cause of the issue. They were not causing issues with Collector while online (before attempting to download for offline editing), and did not cause problems with the Map Viewer nor Operations view. For whatever reason, these additional copies of the hosted feature layer only caused problems when attempting to download for offline editing in Collector. May be a bug.