Guideline in burndown / burnup missing

Summary

The Guideline for burndown / burnup charts is missing

Steps to reproduce

Create a milestone with start- and end date, assign issues and look into the milestone

What is the current bug behavior?

No Guideline displayed

What is the expected correct behavior?

Seeing an guideline

Relevant logs and/or screenshots

Screenshot from today (milestone start) - the green line works well, the gray one is missing completely

Tested on actual chrome and firefox (FF without extensions, Chrome with stuff like adblocker, nothing in my browser console)

Output of checks

This bug happens on GitLab.com ;)