Seen disappearing action bars too, just couldn't nail why or what happened.

Sincerely
~ Kongdej

I had three spells firing off in relatively quick succession. Titan's Breath (from young demon)(all enemies blown back a tile, knocked down, Mass Curse, Horrendous(Hideous?) Wail (awesome creepy scream that does damage/half if resisted). The Champ casting the Mass Curse would end up with his action bar missing. I do not know if it was because it was his spell or because of the wail, or some combination of the affects. It happened frequently, but not consistently. The AI was sending cannon fodder ... heroic defenders my way and out of five tactical battles, it would happen twice. The mass curse was fired first, then the scream. When it cycled back to the Champ that did the curse then at times, no action bar.

I figured out the 'auto' play stop work a round because my Champ started getting wailed on and I was hoping for a miracle, and behold, I got one. The work a round worked consistently. 

Yea, this is a known issue that's in the bug list. I've observed it mainly with auto-cast spells at the start of combat, but it would make sense that it's just any spell going off that has its own entry in the initiative queue. Should be fixed soon.