Required Property not Working in Site but Working on Design Interface.

I am 2 years late to the Question, but thought of putting my experience to help the folks who land here in future
I faced the same issue recently

Problem:
A section Interface was to be made visible on the Site to say User A, however the contents of the Interface was referencing an object whose Viewer Group did not include the User A

Solution:
I added User A to the Viewer Group of all the Objects which were referenced used by the Interface.

I am 2 years late to the Question, but thought of putting my experience to help the folks who land here in future
I faced the same issue recently

Problem:
A section Interface was to be made visible on the Site to say User A, however the contents of the Interface was referencing an object whose Viewer Group did not include the User A

Solution:
I added User A to the Viewer Group of all the Objects which were referenced used by the Interface.