`env['omniauth.auth']` is nil when customizing `callback_path`

I have a Rails app that "lives" in a subdirectory. To make sure the user gets to my app after authentication, I've added the following to my config/initializers/omniauth.rb:

callback_path: Rails.application.secrets.path+'/auth/ldap/callback'

which works fine so far. However, now in my callback controller, env['omniauth.auth'] is always nil

Any Ideas to why this is happening?

I have an almost identical setup in another App (that's served from a Root directory) which works just fine.