SharePoint security broken between Microsoft 365 group and SharePoint group

Hello,

The most interesting detail is that "Check Permissions" returns "None".

In a Teams-connected SharePoint site, Microsoft 365 Group members should automatically inherit SharePoint permissions through the site's Members group.

Since the issue only affects 4 users, I would focus first on the affected accounts rather than on the Teams/SharePoint connection itself.

I would check:

• whether they are actually members of the Microsoft 365 Group in Entra ID;
• whether the issue can be reproduced in a brand-new test Team;
• their total number of direct and transitive group memberships through Microsoft Graph;
• whether there are any differences compared to a user who works correctly (UPN history, guest/member status, synced account, etc.).

Have you tried removing and re-adding one of these users to the Microsoft 365 Group, then waiting for membership propagation before testing again?

The fact that they belong to more than 1,500 groups is a credible lead. While this is not a documented SharePoint limit by itself, Microsoft does document that authorization behavior can become unpredictable when the total number of group memberships becomes very large.

If "Check Permissions" still returns "None" after confirming that the user is a valid member of the Microsoft 365 Group, I would investigate further into either a group membership evaluation issue or an anomaly specific to those user accounts.

Hope this helps point you in the right direction. Let me know what you find.