Pre-release packages are visible in Package Manager regardless of whether Pre-release packages are enabled or not. Refer the attached video.

Steps to reproduce:
1) Create a new project
2) Window > Package Manager > Advanced
3) Enable checkbox "Pre-release packages"
4) Uncheck "Pre-release packages" checkbox

Actual result: Pre-release packages are visible in Package Manager regardless of whether Pre-release packages checkbox is enabled or not

Expected result: Pre-release packages should not be visible in Package Manager if checkbox is unchecked

Occurring on: 2021.1.0a1, 2021.1.0a8

Note: Enable Pre-release Packages is a new feature that is introduced in 2021.1

Environment: Both Windows and Mac