Hm, not sure. This is a kind of side effect you wouldn't expect and would easily overlook. A proper, very long Stage 3 trial could definitely miss it. Generally women's health issues are often overlooked, especially one that is quite mild. That's why we have Phase 4 trials, amongst other things.

This one is especially difficult to pin down because it's a one off. It's not at all evident that longer phase 3 trials would have caught an event that happens at most a month after vaccination.