A one-way air valve does not solve every stack pressure issue

A fictional drainage installation has intermittent trap-seal problems after a new branch is added. A worker proposes fitting an air-admittance valve and declaring the entire stack problem solved. No one has established whether the relevant event involves negative pressure, positive pressure, an obstructed route or another defect. The connection geometry and actual vent arrangement also differ from the old drawing. This written case opens no sewer and performs no pressure test. Explain the particular function of an air-admittance valve, which evidence remains necessary and why a familiar device name cannot replace assessment of the actual flow, pressure and drainage-gas control functions.