Mediamarkts Shop-software is the worst piece of shit ever, so I would think its that. I have nothing but trouble everytime I am so stupid and buy some from them because I am to stupid to remember the last time I felt for them.
I agree that the MediaMarkt Shop is not the greatest BUT different from other shops you at least get a pretty high chance of catching a limited offer/edition, because they always pop up again a short time after. I don't know if it's cancellations but at least gone doesn't mean gone with them. It's just annoying that they can't make the buy button disappear.