SharePoint security broken between Microsoft 365 group and SharePoint group

Hi Coralie!

Thanks for you last message! Here's my answers:

• whether they are actually members of the Microsoft 365 Group in Entra ID;
  ANSWER : Yes, actual member of the Microsoft 365 Group under https://portal.azure.com 
  
  
• whether the issue can be reproduced in a brand-new test Team;
  ANSWER : From an older SharePoint site, the invitation worked.  From a recent site, strange result (see  my last answer below)
  
  
• their total number of direct and transitive group memberships through Microsoft Graph;
  ANSWER : Don't know through Microsoft Graph. From Azure : group membership : 1 622. 
   
• whether there are any differences compared to a user who works correctly (UPN history, guest/member status, synced account, etc.).
  ANSWER : Did not search enough on that subject, but it is something to look into.
  
  
• Have you tried removing and re-adding one of these users to the Microsoft 365 Group, then waiting for membership propagation before testing again?
  
  ANSWER : Yes, did that. Initially, the user did not even see the team under Teams, so I remove completely her access using Admin SharePoint, and gave her access again.  Did receive the email from Microsoft, still cannot see the team under Teams BUT now have access to the SharePoint ... ?!? Furthermore, I added her to a new Microsoft 365 Group, she received the email, have access to the SharePoint BUT still cannot see the team under Teams.  Very strange, different from what happened in the past.  Will look again after the week-end.