I just resolved a similar issue with the help of Esri Support. In case it's helpful I am copying their summary of my case and how we resolved it, below. Good luck!
To recap our conversation, you were experiencing an issue where a layer's settings were set to disable editing, however members in your organization were still able to edit them. When viewing the layer in Map Viewer, they were seeing the message "Editing is disabled but you have privileges to edit this layer".
You confirmed that your organization has custom roles. We found that the "Update" privilege under Administrative Privileges -> Content was turned on. This privilege allows users to "update and categorize content owned by all organization members, and edit data in all hosted feature layers and hosted feature layer views, even when editing is not enabled on those layers" (Portal for ArcGIS - Administrative privileges)
We turned off that privilege in the custom role, and the users were no longer able to edit other feature layers when editing was disabled. Note that this "Update" privilege is different than the "Edit" privilege found under General Privileges -> Features.
