I get loads powder coated I drive past at least 3 to go to the one I use. They bake off first in an oven and it’s a much better finish than the others I have used. I would say prep is absolutely key as with all processes.
I’ve used some in the past that have a fabrication setup right next to the powder line cue lots of grit in the powder.
I would be gutted if something I sent came back like that. If they are letting customer's stuff leave the workshop in that condition then I think it says something about their quality control not just their workmanship.
Showed your pics to the Powdercoaters we used when I was working. They reckon its oil from their air supply. We never had any problem like this from them.