SharePoint/OneDrive and Shared Mailbox access permissions

[robintech]

Recently tightened up permissions and defaults for sharing so now Shared Mailboxes can't access newly shared files as the previous "Anyone in Org with the link" is now "specific People/Groups" files shared with minibuses@ can't be accessed by Alice@

 

Ideally we would convert these mailboxes to SharePoint Groups but there doesn't seem to be a clear way to do it without losing everything including calendars and email or attempting to manually copy them over to the SharePoint Group

 

From what I've read Aliases suffer the same issue, there was something about adding an shared mailbox to a SharePoint group but looks like they discontinued it a few years ago.

Further there are external O365 companies sharing files with shared mailbox addresses which then fail in the same way , although I don't know if that would work with a group either.

 

Has anyone else gone down this path as seems like it should be more common