You’re not cooking it, just steeping it like a tea. So, it does evaporate, but not as quickly as in a sauce. Mulled wine from a outdoor market stall that’s been open all day can be pretty weak, but that’s evaporation that is given the chance to occur over hours instead of minutes. If the alcohol loss still is a concern, brew it cold then heat it quickly with a steam wand to serve. Hope that helps!
You can do this with white wine too! For example, it’s called glüh wein in the Mosel and is most often made with Riesling. It has body to hold the spices and acid to balance any added sweetness.