Sending email to alias of office365 group not delivered to inbox

Hello,
I'm experiencing strange scenario, we have just switched one of our domains from google to microsoft.

 

We have o365 group on microsoft

groupmail@domain.COM

And on google there is a group

groupmail@domain.BE

Where is forwarding to .COM.

If I send e-mail to group@.COM, it ends up in group mailbox.
If I send e-mail to group@.BE it is forwarded to group@.COM but ends up only in my private box and does not appear in group box.

Can someone explain this to me? 
Is it also possible to trace messages to o365 groups?