I've only noticed this with bosses so far, but here's what happens. I'm playing Beast's Wrath on madness.

I get to the Beast
My Golden Son gets killed by him
he "revives", sort of.

The character is invisible when he revives. His circle is there, the buttons to selec his attack are there, everything else seems normal.

With him invisible however, I can't attack. I can't do anything. It's essentially stuck waiting for him to take a turn, which doesn't happen. The only option is close and restart the game.

This happened 3 times in a row so I gave up using him on that level.