What's New page is not loading for Gitlab with relative URL

Summary

The What's new page is unable to load when gitlab is configured with a relative URL (external_url, https://docs.gitlab.com/omnibus/settings/configuration.html#configuring-a-relative-url-for-gitlab)

Steps to reproduce

1. Click on the question mark icon on the top right
2. Click "What's new"

What is the current bug behavior?

The layer displays, but the loading indicator displays forever.

What is the expected correct behavior?

The content of the what's new page should be displayed.

Relevant logs and/or screenshots

GET https://example.com/-/whats_new?v=1234

The configured path /gitlab is missing.

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:
Current User:   git
Using RVM:      no
Ruby Version:   2.7.2p137
Gem Version:    3.1.4
Bundler Version:2.1.4
Rake Version:   13.0.3
Redis Version:  6.0.12
Git Version:    2.31.1
Sidekiq Version:5.2.9
Go Version:     unknown

GitLab information
Version:        13.12.1
Revision:       bec931306a7
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     PostgreSQL
DB Version:     12.6
URL:            https://example.com/gitlab
HTTP Clone URL: https://example.com/gitlab/some-group/some-project.git
SSH Clone URL:  git@example.com:some-group/some-project.git
Using LDAP:     yes
Using Omniauth: yes
Omniauth Providers:

GitLab Shell
Version:        13.18.0
Repository storage paths:
- default:      /var/opt/gitlab/git-data/repositories
GitLab Shell path:              /opt/gitlab/embedded/service/gitlab-shell
Git:            /opt/gitlab/embedded/bin/git