simultaneous turn mode

That's the problem; it doesn't turn off at the right moment when it should.

If the PEST mod is installed, simultaneous turn mode will never turn off; that's the whole point of the PEST mod. And you'll be able to play like in an RTS (i.e., you can steal an artifact from right under the enemy's nose while they're thinking or communicating and then attack, or run away). If you're playing without the PEST mod on the original Heroes 5, simultaneous turn mode will eventually turn off as soon as there's a small distance between you. (I've repeated the first message, but using different words.)