I found the issue myself. I was under the impression that the categories were ordered by ID. They're actually ordered by... the order. I rearranged them in the category manager and all is well. I'm just posting this here in case someone else spends needless hours on this. (I hang my head in shame.)
Sorry, I missed that questions. And Yes. the categories are ordered by ordering. So you just need to go to Events Booking -> Categories, re-order these categories to the order you want
