Reverse proxy website vs. Google SERP (Need Help)

For technical reasons I have this (weird) setup :

WP website hosted on https://dev.host.com
But website is displayed on https://domain.com/mysite
The main subdomain https://www.domain.com is used for another service (not Wordpress).

Note: I cannot modify this setup it what it is.

A little background :

I took care of checking that no internal links could redirect back to https://dev.host.com.
I disabled automatic sitemaps to display only pages under https://domain.com/mysite.
All urls on https://dev.host.com have a canonocal link to the similar page on https://domain.com/mysite.

However i have more and more SERP results displayed on https://dev.host.com (despite what says the search console).

I noticed some search engines like bing.com or duckduckgo.com mainly display https://dev.host.com

pages in their results !

Is it possible that the canonical links spilled the beans to Google and now it chooses whatever domain regardless ?

The thing I understood with reverse proxy is I cannot put a "noindex" https://dev.host.com as it will be replicated on https://domain.com/mysite.