[rayjay]

In my experience, the thrashing normally starts immediately and lessens as the minutes go by and the parasites have been expelled. Also, any I've treated end up lying on the bottom and I have to nudge them to get a reaction. This happens either from the start if no parasites are a problem, or, after expelling the parasites if present.
It is a good sign that the male is eating, but hopefully the female will soon follow.
Keep in mind that the male eating could possibly be a coincidental factor at the time of the dip and something else may be really the problem, OR, hopefully, there is no actual problem at all and you haven't been able to detect the seahorses hunting the pods available at night when they are much more active than daytime.
A pH of 7.9 won't stop seahorses from eating and in fact, many homes, especially in winter, have stale air supply so that proper gas exchange is restricted and thus pH remains at a lower point. This can happen also in summer when a home is shut up to allow the AC to be used.