As somebody selling on Amazon MWS, this might be entirely Amazon's item matching issue - when you are uploading your item feed to Amazon, items on your feed are mapped automatically to Amazon's ASINs (basically their own IDs), and that matching often ignores when you specified pack quantity. I.e. sometimes your 6-pack gets matched as a single item and sometimes a single item gets matched as a 6-pack. They have that issue ~8 years already; I guess to them it's more important to invest elsewhere than to better algorithms...