This is working as designed. As referenced in the documentation functionality for any Studio view level settings such as Required, Enter Once, or Hidden is not available in the Modern UX.
See New User Experience: Compare with Classic CA PPM - System Administrator for more details on Studio and other differences between the Modern UX (Clarity) and Classic PM.
As workaround, you could try limiting the edit for this specific field through new security/through the rules in the Modern UX.