Amazon keeps giving different SKUs/Variations the same ASIN

This has been happening on dozens of my listings whether I try listing with the Excel flat flat or online with add product. The products I am selling do not have UPC, GTIN, or EAN codes which is why I have a GTIN Exemption and Catalog Approval for this brand.
The variations are similar. an example would be a listing with 4 variations each one being a product that is 2 inches wide but 5, 10, 20, 30 feet long lengths.
When I use the excel file I make sure to fill out as many different fields for each sku so that amazon knows they are different products.
I have reached out to seller support many many times but they can never tell me why this is happening or give me a way to prevent this from happening. They just say to delete the listing, wait 24 hours, then list again which doesn’t work.
Does anyone here have any experience with this and knows how to solve this issue?