Cannot assign multiple reviewers

Summary

This post announces that multiple reviewers are allowed since 13.7, https://about.gitlab.com/blog/2020/10/13/merge-request-reviewers/

We are running CE 13.11.

Yet we cannot add multiple reviewers.

Have tried using the reviewer drop-down, this only allows selecting one reviewer, selecting another removes the first

Have tried using the / command with multiple @ people, this only adds the first @ person.

Example Project

I don't have any projects on GitLab.com with multiple members.

I'm seeing this on a self-hosted CE install.

What is the current bug behavior?

Adding a second reviewer replaces existing reviewer

What is the expected correct behavior?

Second reviewer is added in addition to existing first reviewer

Output of checks

I do not have access to logs etc. for this instance, I am not an admin.

Querying if I can get them.