I've been trying to reproduce the issue you've reported, but so far, assets that are in multiple asset groups are still showing up in the report that is generated when you click an asset group's name in the Asset Group Overview widget.
There are some assets that are excluded from these reports though, which could explain missing assets:
- Assets of any state other than active, this built-in report uses a where clause that looks specifically for assets that have the active state, this is the case for most built-in reports, as the main usage of states is to exclude assets from reports.
- If you have a Lansweeper version older than the latest release version 6.0.0.65, assets of the latest Windows 10 build may be filtered out, update instructions can be found here.