Advanced Protection is that ultra‐secure mode that requires hardware security keys to login and prevents third‐party apps. Here’s a quote from its advertising copy:

> Protection against malware is built into Google Chrome, but Advanced Protection performs even more stringent checks before each download. It flags, or even blocks you from downloading files that may be harmful.

Sounds an awful lot like what’s going on here. It would explain why nobody’s been able to reproduce it—almost nobody has AP turned on. I imagine the number of people

1) with AP turned on

2) using a third‐party mail client (mail.app)

3) viewing plaintext mails instead of HTML mail where the link rewriting is less obvious

must be close to zero.