Arrow down-image is missing at edit merge-requests for Gitlab with relative URL

Summary

When editing a merge-request the arrow down-image at choosing the target-branch is missing when gitlab is configured with a relative URL (external_url, https://docs.gitlab.com/omnibus/settings/configuration.html#configuring-a-relative-url-for-gitlab)

Probably related to #356903 (closed).

Steps to reproduce

1. Go to a merge request
2. Click "Edit"-button
3. Look at the right end of the target-branch-dropdown.

What is the current bug behavior?

Arrow down-image is missing.

What is the expected correct behavior?

Arrow down-image should be visible.

Relevant logs and/or screenshots

GET https://example.com/assets/chevron-down-123456.png

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