I have been reading through Shopify community posts for the past months to find a solution for my problem, but up until now I was not able to find any.
So here goes:
At this point I'm out of ideas.
I read about stock being reserved when payment was handled here: https://community.shopify.com/c/Shopify-Discussion/Holding-an-item-on-checkout/td-p/526964 but I suspect that is when e.g. VISA card or bank transfer is being checked, not while being on the payment page.
Anyone has anymore ideas?