I am seeing something very similar.
I am trying to integrate Bitwarden in my existing Docker setup which has Traefik as the reverse proxy.
So far, in general, everything seems to be working, except for the admin page.
Navigating to https://bitwarden.mydomain.com/admin redirects to https://bitwarden.mydomain.com/login which is handled by the web container and thus errors out.
Navigating directly to https://bitwarden.mydomain.com/admin/login kinda works in such that it returns the HTML page of the login page. But it is still broken, because all references to CSS and Javascript files start with a slash, i.e. it tries to get them from https://bitwarden.mydomain.com/lib instead of from https://bitwarden.mydomain.com/admin/lib
I have verified that globalSettings__baseServiceUri__admin is set correctly.
Is it possible that the Admin area simply can’t be hosted on a sub path?
I have since changed the configuration so that the admin area is available at https://admin.bitwarden.mydomain.com and now it seems to work.