MBS fails when a 404 is returned from a product page query. Instead it should assume that the stream suffix should not be added and proceed as normal, with debug output denoting this.
Side note: In the problem case, the product pages returns a 404 with valid json output:
{"detail":"Not found."}
Commit fae378b7 fixes this issue
Commit a5ea3033 fixes this issue
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/mbs/issues/1663
Please continue any further discussion there.