I used Glidden Kitchen and Bath paint decades ago, in a bath that doesn't have ventilation, so it is remarkable that there hasn't been any mildew or mold with the exception of from a leak around a window (on the wall). And the paint looks fresh still. Of course, back then "primer and paint in one" wasn't available.

I don't know if a combo product will or will not perform as well over time as separate applications. When I reno'd my kitchen 6 years ago, I made the effort to prime the walls separately - it didn't take that long. Clearly your contractor wants to save time and effort.

I hope that you are improving your ventilation (out through the roof, not into the attic!!) so you shouldn't have black marks forming from your showers.

Is adding a vent fan an option?

Perhaps over the window?

Sort of attack the problem ...not the symptoms

Paint and Primer in one doesn't mean this paint can go directly over the substrate without it being primed correctly. It's a misleading statement on the paint cans.

https://www.benjaminmoore.com/en-us/interior-exterior-paints-stains/product-catalog/abs/aura-bath-and-spa-paint

The statement just means that if your primed and painted walls were pale yellow and you wanted to paint it beige, you would not have to prime it white first before you paint the beige.

You can use the same Aura paint on the ceiling...........BUT YOUR CONTRACTOR MUST PRIME THE CEILING FIRST IF IT'S NEW MATERIAL.

Tile the ceiling over the shower. it will make a huge difference. Ceiling fan (which is outside of shower) can help manage steam. I have never had a problem.

Growing up, long before exhaust fans, rule was, open the window after you bathe or shower. Close the bathroom door, go and put p j’s on, then close the bath room window. Never saw a speck of mold, or peeling paint.

First address the venting there is no paint going to handle steam day in and day out without good venting. You can use the same paint on the ceiling and BTW get a better painter IMO no such thing as primer and paint in one can and especially new drywall since it for sure needs to be primes and often 2 coats of primer.