Hi @YObaidat,
Sorry for the late reply.😊 The reason why it requires ArcGIS Sign in is because there's a private layer from your ArcGIS Online Organization. To view that private layer, viewers will need to sign in.
"even if they're already signed in to ArcGIS Online within the same browser session" - Do you mean signing in to ArcGIS Online in Power BI? If that's the case, viewers shouldn't see the sign in page.
Please let me know if you have any other questions. Thank you!