Hello Daniel,
I tried to reproduce your issue but unable to do so as I can successfully update network under SQL Authorized networks. Going forward, can you please share the screenshot of the error message observed?
Moreover, you may want to try gcloud command [1] to update authorized network for your SQL instance.
Thanks for your feedback. The permission warning on console looks like a cosmetic bug and you may want to raise it via Google public Issue Tracker (PIT) platform [1]. PIT meant for reporting and tracking customer issue, bugs and feature requests.
[1]: https://issuetracker.google.com