If a tag existed and was deleted, asking for builds in it will return empty list. This is not consistent with behaviour for tags never existed in the first place.
PR #928
Commit 898de96d fixes this issue
Metadata Update from @mikem: - Issue set to the milestone: 1.17
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/koji/issues/926
Please continue any further discussion there.