Does work added to a private project by a public entity require additional prelien paperwork?

I had a contract with a contractor to do street and underground improvements for a lot split project. During the project the city contacted the contractor and requested additional plumbing to the job. This additional work did not benefit my property and a separate contract was agreed on between the contractor and the city. I was not involved in this additional work agreement nor was I made aware of the cost of same which was over 40K . When the job was completed (done by a subcontractor). the city refused to pay a portion of the additional cost (almost 6K). The subcontractor has since filed a mechanics lien against me which includes this additional 6K. My question is can the subcontractor do this without sending an additional prelien notice for the additional work which was on a different contract, a different client and a public entity?