This has happened on a few occasions. I would be fighting multiple mobs, in the center of them. I would then activate Reckless Frenzy. It would be fine for a few turns. But then my character would stop fighting in mid battle. No attacks were blocked by the way. Then chests would pop. Yet you could here the sounds of my character still fighting. My HP would still be deducted during this.
I would lose the chests. I was unable to open any.
This has happened when I fought sky squids, harpies and just recently, Cadmus.