This article lists known issues with Identity-Aware Proxy (IAP).
Signing in users with external identities from Safari and Brave
A recent browser change is preventing some users from signing in with non-Google identities on Safari and Brave. Google is aware of this issue, and working on a fix.
To mitigate this issue:
Safari: Instruct your users to disable Prevent cross-site tracking. This setting is located in the Settings > Privacy section.
Brave:: Instruct your users to Enable all cookies. This setting is located in the Settings > Shields > Cookies section.