Burndown charts never goes to zero

Summary

Burndown charts show the milestone progress but the graph does not get to the floor, and make it look like it is incomplete

Steps to reproduce

Open a new milestone, create issues, close them. The line does not go to zero

What is the current bug behavior?

The line shows 3 open issues

What is the expected correct behavior?

There are no open issues

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com