User (sys_user) fields are syncing to what is populated on associated HR Profile (sn_hr_core_profile).
This is working as designed.
This is occurring because (Synchronize fields to sys_user/Synchronize fields to sn_hr_core_profile) business rules are designed to run upon updating a (sn_hr_core_profile) record:
These business rules were designed to keep the (sn_hr_core_profile) & (sys_user) record in sync as HR users do not usually have access to update the sys_user table.
The Script include function (hr_Utils().syncProfileFields) in these BR's call the hr_Utls Script Include below:
Adding the fields you desire to no longer be synced to the (profileExclusionList) of this script include will ensure that these fields are no longer synced when updating an HR profile