Hi Tero,
In the company where I work, we have experienced the same issues as you have. When I read your post it seemed like I was re-reading a chronology of my exact experience.
I was not able to get useful answers from HubSpot or Meta support on this topic. Both seemed to blame the other and in the end, we gave up trying to use the Hubspot-Facebook event optimization integration for the time being.
I am sorry that I do not have a solution for you. However, I thought it is important to let you know that you are not the only HubSpot client using Meta Ads and experiencing this issue. I think this feature was brought out too early before it was fully funtional.
Best,
Jonathan Fichman
Global Head of Paid Marketing