Thanks for your reply. Upon further investigation I noticed that it most likely is not a plugin issue, as a variations of Meta pixel tracking solutions seem to track the values correctly on AddToCart, InitiateCheckout, ViewContent and Purchase events.
There seems to be a mistake though with WHEN a Purchase event is fired (with the wrong value). Looking at the Events Manager, it looks like per actual Purchase, two Purchase events are being tracked: one on the order confirmation page (https://startkraft-rawfood.de/checkout/order-received) with the correct value, and one on the checkout page (https://startkraft-rawfood.de/checkout/) with the incorrect value.
This issue also doesn’t seem to occur every single time, meaning some recorded purchases only have one tracked Purchase event.