Did the port tacker tack in the two-length zone?
Perhaps, when you overstood the mark, you were telling the skipper to bear off EARLY to get on the layline, so rule 18.3(a) would apply?
If you overstand the mark then 18.3(b) apllies.


18.3 Tacking at a Mark
If two boats were approaching a mark on opposite tacks and one of them completes a tack in the two-length zone when the other is fetching the mark, rule 18.2 does NOT apply. The boat that tacked

(a) shall not cause the other boat to sail above close-hauled to avoid
her or prevent the other boat from passing the mark, and
(b) shall give room if the other boat becomes overlapped inside her, in
which case rule 15 does not apply.