In the previous version of Collector, we had a default offline basemap sideloaded (https://www.esri.com/arcgis-blog/products/collector/field-mobility/using-your-own-basemap-layers-with-collector-for-arcgis/) on all of our iPads. When users downloaded a "Map", the default was to select the offline base map, as our jurisdiction is 500000 acres and it's simply not possible to download a 4 GB basemap over ANY kind of connection. This worked flawlessly for years. Now, with the new version of collector, you can still use the sideloaded basemap, but only AFTER you've downloaded the original basemap when you pulled in the map. (https://community.esri.com/message/824131-re-collector-maps-offline?commentID=824131#comment-824131: " Using a sideloaded TPK during download isn't supported at this time").
This has effectively eliminated our ability to use Collector.