Solved! Go to the solution
This is an accepted solution.
I just had this problem and it was solved by Shopify Support's suggestion to check if any of the barcodes have been stored with a leading 0. When I removed the leading 0, the UPCs were recognized.
What's a little weird though is that prior to making this change, typing in the UPC code (without the zero) brought up the product but not the specific variant even though the UPC barcode is per variant.