Look, guardian spirit shouldn't be affected by battle fatigue because it wasn't in s15.
Now, it's not "either all not affect or all affect jajjajajaja".
Think of Guardian Spirit as a death mechanic. It's effect only triggers on death. And what does death do? Remove all buffs/debuffs. And what is Battle Fatigue? An invisible debuff. So it gets removed on death and THEN the effect triggers. Get it now?
Then again, blizz might have had a different reasoning.