I've played a fair amount of co-op and never noticed this issue with any build. Is this possible it was a one time bug, or have you reproduced it multiple times ?
If yes, then that's definitely a HUGE issue, yeah.
Starting combat appropriately is definitely an issue atm. Not only with the case that is described here, but also for example when you revive a character on a damaging surface such a fire. That character will start taking damage in real time until he enters combat... which sees to sort of arbitrarily happen when moving or attacking (might have something to do with the sight of enemies.
The bottom line is that the way combat is initiated should be looked at again.
