Reproduction steps:
1. Create a new project and import the attached custom package "1.unitypackage"
2. In the Project Window select "Test_character_SpriteLib.spritelib"
3. Notice how in the Inspector Window the thumbnails flicker

Expected result: Thumbnails in Sprite Library Asset do not flicker
Actual result: Thumbnails in Sprite Library Asset flicker (See attached Flickering.mp4)

Reproduces on: 2D Animation 6.0.0-prev.2 - 6.0.1 (2021.1.7f1, 2021.2.0a16)
Could not test on: 2018.4.35f1, 2019.4.26f1, 2020.3.8f1 due to thumbnails not being present in the selected Sprite Library Asset or the asset breaking when downgrading the 2D Animation package

Notes:
-Issue does not reproduce on a new project
-Deleting the Library folder did not solve the issue