It appears that side-loaded basemap TPKs in "new" Collector must match the spatial reference of the webmap's original/native basemap...typically WGS84 Web Mercator..and you are forced to download a default basemap (whether you want it or not) instead of being able to select from TPKs already on the device. So, you have to download basemap tiles plus data, instead of just data. Not quite "new and improved" in situations where connectivity is dodgy.
In Collector Classic, you could select your TPK pre-download, and easily switch back and forth between TPKs having a different spatial reference once a webmap was on the device.
See this link for discussion and "workaround"