Defense is colorless
This.
Defense damage is colorless so Jackal and Falcon lose the Elemental defense debuff.
Defense damage does not color stack so Jackal and Falcon lose that hidden buff.
Defense damage does not get combo bonuses so that buff is lost.
In the end I took Wu Kong 4* 4.70 and Jackal 4* 4.70 off my defense team and replaced with Joon 8/8 5* 2.52 and I am doing much better.
Joon, Alberich and Natalya are three heroes that do not rely on Elemental defense debuff, color stacking, or combos, so they do exceedingly well on defense, even at 5* 2.60 and 3.70 .