Why are some attributes showing as required in the Classic view but not in the Modern UX. Can this be fixed?
Some OOTB attributes cannot be included. If the attribute cannot be edited on the attribute page to set it to required it cannot be set to Required in the New UX.
You do have the ability to make the attributes required on the object/views, however making the attribute required here does not transfer over to the New UX. Only attributes marked as Required on the attribute itself will show as required in the New UX.
Some attributes which are not able to be set as Required are: