MR approvals are notified to Slack as "MR opened" messages instead of "MR approved"

Summary

With Slack notifications on GitLab events enabled, Merge Request approvals are sent to Slack as "[user] opened !xxx [MR title] in [repo name]" instead of "[user] approved [MR]".

Receiving this wrong notification text is however only happening sporadically. We require 2 approvals for each Merge Requests and it might be the case that this bug only happens for the first approval. This description will be updated should I discover a pattern.

Steps to reproduce

1. Enable Slack notifications for Merge Request approval events in repository settings
2. Approve a Merge Request
3. Read the notification in Slack

What is the current bug behavior?

Merge Request approvals are sent to Slack as "[user] opened !xxx [MR title] in [repo name]"

What is the expected correct behavior?

Merge Request approvals are sent to Slack as "[user] approved [MR]"

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com