We have this happen as well, its a major problem for us as the maps this happens to are used in field maps. Field maps silently (except for in debug logs) fails after download a map to the device if one or more layers have not been shared with the recipient.
@RussRoberts your workflow is not quite how I would expect the webmaps to work.
Day One
- Create Webmap
- Share with Group X
- Save Map
Day Two
- Come back to add a new layer a few days later
- Add new layer Y without any sharing setup
- Save Map
> No prompt or warning to the webmap owner that the reciepients of the map cannot access the map.
Group X will get a warning that Layer Y is not available when opening a web map online, that the layer is not available. If opening in field maps via an offline package, the package downloads but will not open.
You have to open the sharing tab and click update to update sharing every time a change is made to the map. Its only one additional step but easy to gloss over when building a map. The map should warn you if the intended recipients of the map cannot access the map contents on save.