We build this way also to get the latest ExB features.
We do get a popup with just an ok or cancel in it as our connects to SAML. We click ok and get in. BUT we have all kinds of issues in the ExB after that. For example if we use imbed widget with a portal source in it the browser goes into an infinite loop of logins and it grinds to a halt and never does display. Looking at dev tools it seems to be trying to send the AGOL creds to Portal (vs the portal creds). So it is not keeping them straight it seems. We did open a bug now on this but I do not have the number.
We also see it sometimes with geoprocessing services and the analysis widget. We see denials in the server logs but have not tracked it down more yet. But seems to send the wrong creds again.
There are some other examples that I am forgetting now. We did try to move all to portal but there were a number of other issues that came up from being older version and some other weirdness.
I know this does not help you but just adding in to confirm this and in the hope it gets some more attention from the ExB team. But note the team did give us some special URL adds to try but they did not work. But just in case look here. https://community.esri.com/t5/arcgis-experience-builder-questions/embed-widget-fails-to-load-if-link...
thanks!
