Adding labels from sidebar removes labels added from `/label`

Summary

Given labels added through /label command and the page has not been refreshed, when more labels are added through the sidebar, the existing labels are removed

Steps to reproduce

In an issue:

1. Add labels via /label ~label_A
2. Note showing "username added ~label_A" appears
3. Without refreshing the page, using the sidebar, add a different label ~label_B.
4. Note showing "username added ~label_B and removed ~label_A" appears
5. Result: ~label_A is removed and ~label_B is added

Example Project

What is the current bug behavior?

~label_A is removed and ~label_B is added

What is the expected correct behavior?

~label_A is remains and ~label_B is added

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

Possible fixes