Does it appear only in listings that has linked an inventory item or not?
No, none of mine are linked to inventory items
To which amount does the quantity "reset"? To the amount the quantity has before or to a random value?
All of mine are unique so the number is "1". It resets to "0" so it does not re-list
Is this listing running or just prepared? Maybe the value is updated from eBay?
I have found that whole blocks of listings will change - active and not. There will be a block of hundreds that are re-set then the rest fine