When the control break is during your turn, it should apply instantly and if not it’s a bug.
I’m pretty sure that control break can be stopped during your turn, but is it always, I can’t say.
Kill a Siren stops control during your turn.
Blow up Siren head doesn’t stop control during your turn. It tend put Siren will at zero, then during next enemy turn the control will break.
That’s a feature, and you should better put care of your targets for example with mingfraggers. I admit that as allies won’t attack your soldiers controlled, nor turrets will do it, the aiming should put them at end of your aiming list.