I think we should close this issue as won't fix since it seems it's working as expected. WDYT?

What would need to be fixed in the future, is to have some warnings when changing a user's group and there are pages requiring some rights that the user no longer has (and that user is the last author for those pages). This is related to the Document Rights proposal: https://design.xwiki.org/xwiki/bin/view/Proposal/Documentrightsmanagementimprovement