Guest Users can't access SharePoint Website if they are also delegated Admin (MS Partner)

Question

Hi everybody,

I found a confusing issue and I'd like to know if this is by design or an bug. I try to explain it:

Let's assume I am a Microsoft CSP. My company name is contoso.

I use the granular delegated admin permissions (GDAP) to administer my customers.
When I access the customer tenant, my username is replaces with "user_" followed by some numbers. This is by design with GDAP. When I sign-in as a partner admin to the customer tenant, then the customer will see in the sign-in logs something like "contoso technican" not "******@contoso.com".
I am also invited as a guest user to that tenant with my account ******@contoso.com. So when I access the tenant as a guest, my username will be ******@contoso.com.

Problem:

I am a MS Teams guest of a Team. I can access the team as ******@constoso.com and I can also access the files tab within the team and downlaod the files. But, when I try to open the files in the Browser with for example "open in SharePoint" or "open in Browser" I got the error message that I don't have permission to access this SharePoint Website and I need to request access.

What happens is that when opening the SharePoint, it obviously changes my UPN from ******@contoso.com to user_<somenumber> which is the GDAP user.