unity2021.2.17 TextMeshPro3.0.6, Shader TextMeshPro/Distance Field ‘DebugSettings’ ‘Mask’ options，Cannot choose MaskHard or MaskSoft. Test useing 2020.3.15 TextMeshPro is working , but switch to 2021 not working.
Since the LTS 2021.3.x version is out, you might try updating to a supported release version to see if it was a bug in the pre-release version you are using.