[Defender] Defender doesn't let me access "/login" when login URL is masked

Whenever I enable Defender’s Mask Login setting with any random slug, a page within my site with the slug “/login” becomes inaccessible. It says “This feature is disabled”, even though the mask login URL slug doesn’t have “login” in it.

We have tried a plugin/theme conflict test, enabling and disabling the setting and changing the mask login URL slug, but none of them helped. Debug.log doesn’t show any relevant errors. Please advice.