Hello,
We just discovered the same issue on ArcGIS Enterprise 10.8 !
When configuring a group viewable and joinable by only "Members of an Enterprise Group", users can't see them even when they are in the group.
It used to work ! The funny thing is that they can see old groups created with older version of Portal for ArcGIS that are configured the exact same way !!!
I decoded the reply from SAML when logging in:
And of course when trying to access webmap from that group, I get 403 error.
This is a big issue for us !!