Trigger variables not shown in multi-project pipelines

Summary

Trigger variables are shown when triggering through the API, but not when using trigger Jobs from Multi-Project Pipelines.

Steps to reproduce

Create a project which triggers a downstream project using the trigger syntax from this page and pass one variable to this downstream project. The trigger variable is not visible in the downstream jobs.

What is the current bug behavior?

The table for trigger variables is not shown.

What is the expected correct behavior?

There should be a table displaying all trigger variables exactly like when the job is triggered through the API.

Gitlab Version

11.11.1