Oauth2 redirect problem with Chromium browser

Nextcloud version (eg, 20.0.5): 21.0.0
Operating system and version (eg, Ubuntu 20.04): UnRAID 6.9.1
Apache or nginx version (eg, Apache 2.4.25): Nginx 1.18.0
PHP version (eg, 7.4): 7.4.16

The issue you are facing:
I have setup a Nginx reverse proxy to use Oauth (via Oauth2-Proxy) to authenticate access to specific subdomains. I use Nextcloud as Oauth provider.
The redirect in Chromium based browsers isn’t working correctly (Firefox works fine)

Is this a thing I can fix on my end?

Nextcloud is setup on: nextcloud.domain.com
The Oauth proxy on: oauth.domain.com
The protected WebUI on: portainer.domain.com

Is this the first time you’ve seen this error? (Y/N): Y

Steps to replicate it:

  1. Setup a Oauth2-Proxy that uses Nextcloud as the Oauth2 provider (e.g. Oauth2-Proxy or Vouch-Proxy, tested both)
  2. Put that Oauth2 proxy on it’s own subdomain (e.g. oauth.domain.com) behind a reverse proxy
  3. Setup a different service behind the reverse proxy (e.g. the Portainer webUI) that requires authentication via Oauth. Authentication setup via the reverse proxy.
  4. Try to access the protected subdomain by login via Nextcloud
  5. The redirect after granting access will fail due to the Content-Security-Policy “form-action” not taking other subdomains into account (only the redirect URI set for that Oauth client)
Refused to send form data to 'https://portainer.domain.com/' because it violates the following Content Security Policy directive: "form-action 'self' https://oauth.domain.com/oauth2/callback".

The https://oauth.domain.com/oauth2/callback represents the exact Redirection URI set in the Oauth settings in nextcloud. If I change that URI to https://portainer.domain.com/ instead, I get the same issue but reversed (It then can’t access https://oauth.domain.com/).