I wonder if this might actually be related to how ADE is common from coronavirus vaccines but not infections. As far as I know, no one has figured out why it happens; but if the immune system reacts differently to things which make it into the body in a general way from things in the lungs, then it could be that injecting respiratory viruses into the body creates a different immune response in a way that coronaviruses end up using to their advantage.

I was wondering about this for a few years, and suddenly have a plausible explanation. I'm not sure if anyone has done the research to look into it, but it'll be easier to find it if it exists now that I have an idea of something to look for.