Arrow down-image is missing at Repository Settings for Gitlab with relative URL

Summary

At Repository Settings (Settings -> Repository) the arrow down-image at choosing the default-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 #332013 (closed).

Steps to reproduce

1. Click the Settings-menu within a project.
2. Click on Repository within the Settings-menu.
3. Click on "Expand" at "Default branch".
4. 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.