When I trigger the ‘showMixedValue’, the text is disappeared, and the mixed checkbox does not have the background.
Thanks for bringing this up, this looks like a bug. Could you report it through the Unity bug reporter (Help/Report a bug…) ?
I submit a bug report Case 1369122. And I build a test project can reproduce this issue.
Open the project with 2021.2 beta and select ‘Test/ShowMixedValueToggle’ from the menu.