Pre-Steps:

1. Install 2021.1.17f
2. Add com.unity.ui.builder 1.0.0-preview.16
3. Add com.unity.ui 1.0.0-preview.16

Note: any other combination probably won't exhibit the issue -- did not try all combinations but so far, seems related to 21.1 w/ both packages installed

Repro Steps:

1. Create a few buttons in a new uxml file in a new project, save the UXML
2. Add a new selector with nothing in it and save the uss file

Result: The buttons' styles should become flat, Buttons do not look like Buttons anymore
Expected: Buttons keep their original style

Notes:
- Does not limit itself to Buttons.
- Reloading the UI Builder does not make the problem go away. You need to physically remove the USS file, start afresh with a new UXML, and then Reload the UI Builder window to get the original styles back.

Bug reported here: https://forum.unity.com/threads/builder-applies-styles-from-a-uss-file-that-is-not-actually-loaded.1157354/