I think it has something to do with Resist Fire aura.
I just did a quick test, and Tainted overwrites Resist Fire aura.
First pic with tainted nearby.
Second pic with tainted in town.
So my guess is the Resist Fire aura from Flickering Flame is overwriting the Tainted's Resist Fire aura, and the game is treating the Tainted itself as the aura, and deleting it too.
But I went up to lv 8 Resist Fire on my test pally, and didn't kill my tainted. I removed +skill gear until my Summon Tainted skill was lower level than the aura. Still didn't kill it.
So maybe this interaction only takes effect when you have the aura on your own character? as a way to prevent party members from nuking your minions by using Resist Fire? Either way, got a weird interaction with Resist Fire in that Tainted overrides it even when a Resist Fire skill being used gives more Fire Res.