I'm not sure why you'd put a pre-emergent underneath mulch. That doesn't make sense to me. We always install it on top.
If I am laying mulch, whatever I am laying it on is clean. I've already sprayed out the brush or weeds and removed all the brush or weeds. Then I'm adding a 2" layer of mulch on top. It's not like new seeds are going to blow in, wiggle their way down 2" through my mulch and find the soil before they start germinating. No! The weed seeds are going to blow in and start germinating on whatever they land on - namely the top of my mulch!!! So that's the area I want to protect.
Applying pre-emergent underneath mulch is just about as effective as laying down weed barrier fabric and then installing several inches of mulch right on top of it. The weeds may not grow too deep. But they'll grow on top of that mulch - no problem.
I don't know. Maybe it's different here in Oregon. The bark mulch we have here is pretty fine and weeds grow on it no problem. It rains all the time here. All you need is a little bit of soil, mulch, dirt, whatever - and the weed will grow on that no problem.
I find weed fabric to be totally useless, unless you're installing something totally inorganic on top of it (e.g. river rock). And I find pre-emergents applied UNDERNEATH mulch to be equally ineffective.