UNeditable field?

I have a field in a client table that is the key/label and using a REF for my master contacts table. It is checked as editable. However, I can only input data in the field from an initial entry of a new record (seeing the values being referenced from the master table). If I attempt to make an edit after saving/uploading to this one field, it is locked. However, all other fields are available for edits. Thank you!

This is the expected behaviour for Key-columns. They can not be edited because doing so would break references stored in child-tables.