When using Red Hood’s skill to create Fox minions, using a banner battle item makes the “Resist” message show up on the heroes with fox minions.
The “Resist” message showing up is just a visual bug though. The banner effect still applies to the heroes, so thankfully the banners still work.
Screenshot below. Note that the effect is still casted, yet the resist message is showing up (and there’s very clearly no enemy here that reduces mana).