Keycloak claims to handle authorization, but the part that it handles goes only up to enumerating what accesses you have.

Checking those accesses and allowing or restricting the usage is up to your system.

I’m no expert, but Keycloak is just doing authentication (does the user have a valid account?) for you. For authorisation (is the authenticated user allowed to access the site?) you could put Pomerium in front of your apps and authorise based on user groups for different paths.

Not sure about portainer though, that might be a bug?

I have no experience with portainer, but some apps have an option to disable new user registration, and that's what I would recommend first. (I do use Keycloak myself too)

I use specific groups within KeyCloak to define what applications I want a user to see. However, I am using Cloudflare Zero Trust and passing the group claim.