Death Minotaur used to cast a Doom & Gloom bubble that boosted death damage. Since the bubble update, however, he just casts an invisible spell and there's no visible bubble, though death damage effect is still in place.
We had a Shadow Seraph healer on our team who used rebirth in the Hades fight. As expected, this caused Zeus to cast a "hesitation" minotaur at her since the spell is over a certain number of pips. However, after Shadow Seraph produced absorbs for all our teammates, Zeus cast a second minotaur in the same turn. One spell, one minotaur, we in the team were in agreement that that should be how this works. Two minotaurs seemed glitchy.
#1 is because no one thought to put in a texture for it. Bubbles used to just use the base school texture, but with the new KR bubbles KI recoded the bubbles to look for an assigned texture. Death and Life damage bubbles were probably overlooked because they cannot be learned, thus they do not have a texture and are invisible.
#2 is intentional according to the wiki. Zeus uses an extra minotaur on shadow spell users. He's probably one of those guys trying to get Shrike banned from PvP instead of thinking of counters.