Are any of the members marked as hidden from the GAL? Members hidden from the GAL will get the email, but will not show up in the member list or when expanded if they are hidden
You can check if they are hidden from the GAL by checking the msExchHideFromAddressLists
attribute in AD or with the following command in Exchange Online PowerShell:
Get-ExoMailbox -Identity ******@domain.com -Properties HiddenFromAddressListsEnabled
Or you can check in the Microsoft 365 Admin Center
- Go to Users > Active Users
- Search for and open the user
- Switch to the Mail tab
- Check the setting under "Show in global address list"