Hi
'0' is not added, it is a separate field in product-category relationship, called "rank", so there is nothing wrong with that.
Your problem is something else, make sure:
1. the shop you are browsing has access to that category branch (JAM > Shops > "Your Shop" > Catalog Tab). The category must be "reachable", so it or its parent have to be assigned to shop (that is how you give a shop access to catalog)
2. product and its SKU is created/imported (SKU also must exist as this is the actual unit which is bought).
3. product has to be assigned to a category in the branch assigned to shop (you already said you assigned it to product, make sure category's branch is assigned to shop, see point 1)
4. product SKU must have price for that shop in correct currency (each shop has its own price list in specific currency, so make sure that price exists)
5 product SKU must have inventory record even if it is backorder/preorder and inventory record must exist in a fulfilment centre. This has to be like this as you can have multiple fulfilment centres and that affect the shipping and cart splitting mechanism (used by marketplaces and nationwide shops).
6. make sure that shop has access to this fulfilment centre, similar to catalog assignments (JAM > Shops > "Your Shop" > Fulfilment centres Tab).
7. (you already done this but for sake of completeness) reindexing (JAM > System > Search Indexes) must be run if you do global changes to products like imports, if you edit/create products in JAM single product reindex is run automatically. HOWEVER even if your index is up to date, there is also caching in storefront so some times it takes a little while for the product to appear is some searches (especially if you just refresh). Alternatively you could force cache evict from JAM > System > Cache Monitoring
Regards,
YC team