Slack messages never show up from Gitlab triggers

Summary

We have several repos in Gitlab which were created in older versions of Gitlab (I do not recall what versions, but they're several years old at this point). We're currently on this Gitlab version: GitLab Community Edition 9.5.8. These repos are unable to send messages via their Slack integration, while other newer Gitlab repos are able to.

Steps to reproduce

Create a Gitlab repo in some older version of Gitlab. Migrate up to 9.5.8. Setup a Slack integration and you won't see any messages in Slack when Gitlab triggers fire.

What is the current bug behavior?

When triggers get fired from Gitlab for some repos, no messages show up and we see nothing but 200's in the gitlab logs regarding this. We have no visibility to any Slack logs and so can only see the lack or success of messages as a by-product of triggers.

To make this issue even more odd, we've noticed that if we target another instance of Slack with these supposedly misbehaving repos, messages will show up.

As a work around we noticed that if we explicitly set the channel name in the Slack integration screen for each trigger, they'll work correctly against a given webhook. If we omit these they silently fail.

For the newer repos, if we just specify the Slack channel's webhook URL with no explicit channel names per each trigger, messages are successfully sent to Slack.

Screenshot of workaround

Below you can see where the name of the channel for Slack was added to every trigger within Gitlab's Slack integration dialog.

What is the expected correct behavior?

Trigger messages should show up in the Slack channel that's designated via the webhook URL.

Relevant logs and/or screenshots

When I looked in the gitlabctl tail I saw only successful http status 200 + 302's which would seem to indicate that the calls are getting through!