Q: Why does ISO 811 testing fail at the zipper seam?
Water usually finds the stitch line before it finds the film. Needle holes enlarge the path through the tape, the seam allowance wicks water along the thread, and the fold at the placket creates a channel. A taped seam that is not fully bonded, or a tape applied over a contaminated or damp surface, gives a continuous leak path that no coating on the tape can compensate for.
Diagnose by testing three specimens: plain tape, tape with chain, and the assembled seam with sealing. If the plain tape holds and the seam fails, the problem is construction, not the tape. Check stitch density, needle size, thread type, sealing temperature and pressure, and whether the slider body sits on the seam. Test after washing, because heat and mechanical action can lift an edge of the tape.
Acceptance should be judged on the assembled specimen at the pressure the garment claims, with the failure location recorded. Report where the water appeared, since a leak at the slider is a design issue and a leak along the stitch line is a sealing issue.