Label filtering not working for Slack notifications integration

Summary

When setting labels for the Slack notifications integrations, no notifications are triggered anymore. Leaving the labels field blank, correctly triggers notifications.

Steps to reproduce

1. Setup Slack webhook
2. Setup Slack notifications integration
   1. Only set up notifications for Issues
   2. Add labels

What is the current bug behavior?

No notification is posted in Slack.

What is the expected correct behavior?

Being notified about new/updated/closed issues based on the defined labels.

Relevant logs and/or screenshots

This is how we setup the labels. The label is called QA. We've tried different variations of the label (as can be seen in the screenshot). We've tried with single labels, multiple labels, different labels, ... As soon as the field is not blank, no notifications are triggered anymore.

Output of checks

Results of GitLab environment info

We are running an on-premise, enterprise edition: 13.11.3-ee