Turns out in the last Bodhi 3.5 FBR, I'd added fedora_28_stable_master_repomd, but forgot the _testing one.
diff --git a/roles/bodhi2/base/templates/production.ini.j2 b/roles/bodhi2/base/templates/production.ini.j2
index a49cfb306..65ba35911 100644
--- a/roles/bodhi2/base/templates/production.ini.j2
+++ b/roles/bodhi2/base/templates/production.ini.j2
@@ -194,6 +194,7 @@ pungi.extracmdline = --notification-script=/usr/bin/pungi-fedmsg-notification --
{% endif %}