I have never seen this behavior before. I have a user with a hosted feature service whereby the permit type field shows a lock on the data table (figure OCP_1), but the field is editable (figure OCP_2), and the settings allow for Add, update and delete features (figure OCP_3). Additionally, it is possible to edit the feature within the web map.
There is a relationship class / related table, but the primary key is based on permit id and not permit type. I don't have any details about how the feature service was published. My next step is to download and attempt to publish a new service --- I don't want to overwrite at this point.
Has anyone else encountered this issue where the field is locked in the data table, but is editable within a web map?
If you browse to the Visualization tab > configure pop up > configure attributes > does the field show as editable? I did some quick testing, and as I have seen in the past there is a consistent relationship between the lock icon and not being able to edit the field. If this is still occurring I would recommend getting in touch with Esri Support.
Hope this helps,