There's actually no guarantee that Sphinx's debuff is as strong as Mordred's. While we assume it is, most eidolon effects are actually much weaker than the ones provided by Souls and SR+ characters.
And in fact, according to the JP wiki, Sphinx's skill is indeed weaker than Mordred's. Seems to have a lower accuracy too. So you rely on it at your own peril.
But no doubt elemental disadvantage provides a penalty to debuff accuracy as well. It was the combination of Thunder + Sphinx that did you in.