When I had a contract including the objective "Build a new station that has an antenna, has a docking port and can generate power" and used KAS functionality to attach an additional antenna to the vessel, suddenly the objective became unfulfilled, even though it was marked as fulfilled just before attaching the new antenna. Same applies to a similar unmanned probe contract - here the objective to "build new unmanned probe that..." also became unfulfilled after attaching an antenna to the vessel.

See the attached screenshot - topmost objective was marked as fulfilled before attaching the highlited antenna.