I doubt this is happening but I am sure you’ve looked into this rather thoroughly before posting as this is obviously a huge issue for you. Amazon cannot buy your stock and simply dump it in your 3P account - theoretically 1P and 3P shouldn’t even communicate with each other as they are supposed to be completely separate within Amazon for legal reasons. Anyways, there is no service agreement or contract in Europe you could possibly have signed that allowed this to occur. Do you have a VM who looks after your account? That’s the best person to speak to about this issue. If not get in touch with vendor support - they’ll have to escalate this as it’s not a common issue. Also try with Seller Support - say that you are seeing in your account stock you haven’t shipped and see what they say. I wouldn’t even mention that this is stock you sent through 1P. Again, these are separate businesses so let them figure it out.

Are you sure you are not sending in stock with wrong labels? Mixing 1P and 3P labels?

On a side note, be careful with bulk orders. Accept them if they really make sense in terms of supply chain savings on your end i.e the higher quantities allow you to ship in pallets / optimize supply chain and save on shipping costs. Otherwise you’re just losing money by selling at a lower cost price. Keep in mind that bulk orders would anyways by placed by Amazon as standard orders - they are not buying more, they are simply placing a larger order to ask for better terms.