Right now, any imported MARC Bib will update the existing MARC record stored in SRS. If the import is only meant to update an item record, the user does not expect the SRS ARC (and the corresponding instance) to be updated by that imported MARC Bib. The only thing that would cause part of the SRS MARC to be updated is if the user includes a MARC Update action that only acts on specific fields. The SRS MARC update is implicit right now.
On the other hand, to impose the MARC record field protections, a user must include an explicit Update MARC action in the job profile, even if they are not overriding any of the field protections.
These are not ideal or expected behaviors. We need to talk through the user expectations and figure out how to make the job profiles behave as expected. In some cases, it may just be a UI change to make the implicit actions more explicit. In other cases, it may be a behavior change for Data Import
Also consider reviewing and refining Field protections, Override of field protections, and Updating individual fields as part of this feature