True that women have historically been excluded from a lot of drug trials but this isn’t the case for the covid19 vaccines. All trial data I’ve seen include men and women in roughly equal numbers. Actually it was older people who were originally left off this time (most early trials excluded adults over 60yo).
In the relatively small numbers of people in phase 1 and 2 studies (in the hundreds of people) and even in phase 3 where thousands of people may be included it’s still possible to miss side effects.
E.g. due to how rare it is, we didn’t pick up the clotting issue with AZ and J&J until post market surveillance.
It’s possible that any changes to menstruation would have been missed, even if it were included in the trial to monitor impact on menstruation.
Exactly what percentage of women are affected is not yet clear, because at present we’re hearing mostly from women who say they have been affected. But not those who haven’t been (eg myself).
Not to mention that the waters have been muddied by people insisting that they cycles and menstruation have been affected just by being around vaccinated people. Which is ridiculous, there is absolutely no biological basis for that to be possible.
The good news is that there are some researchers actually studying this now - so hopefully we’ll have some accurate data on whether this is a thing or not, and how many women are affected. And how long it might persist for.