trouble is that PoTS tends to show up at around the same time as the vaccine is given (in the same way as autism starts to become noticeable around the MMR time), so it's really difficult to say whether one causes the other (or more likely, is the trigger for worsening symptoms).
DD2 started fainting a few weeks after she had the HPV final dose, which was eventually diagnosed as PoTS, but crucially, once we got to grips with what PoTS was, and how it affected her, we realised that she'd been having mild symptoms for years and years. So in our case, it may be that the HPV was a trigger for her worsening symptoms, or it may be that there would have been something else, and the timing is just a coincidence.
FWIW, DD1 voluntarily got the HPV (as she was too old for the initial roll-out) and DD3 has had both doses as well. We decided that the potential risk was worth it.