Hi, Benjamin,
I’ve just tested it on my server, but it seems to be working fine here – the prices are correctly updated after I click on the “Recalculate” button (in the “Product Price by Quantity” admin meta box). Would it be possible to share access to your site so I could log in and check what’s going on (my email is [email protected])?
P.S. I can confirm there is an issue when creating a new order (i.e., items are removed), so we have to save the order first (before clicking on the “Recalculate” button). I will be fixing that in the next plugin release, however, it still won’t solve the problem in your case.