There are certain OOTB fields which are not allowed to be secured in MUX.
One of these fields is the Resource ID.
In Classic you can set the field to Enter Once, however this is not honored in MUX and this is one of the fields not available for Field Level Security.
How can we secure this field as well as others not available for FLS?
Release : 16.1.1, 16.1.2
Current product design does not have the option to secure these in MUX.
The functionality to partially secure these attributes is being worked for an upcoming release.
It will still allow users to see the values, but the ability to assign edit/update rights to certain groups will be available.