I had an issue related to this thread but as that i several months old, made a new post.
I have a fairly new install on Ubuntu 18.04. All system-level settings default. I installed Slack but snap. It’s at version 3.3.1.
When I ran it (but pressing Super key and typing Slack), the icon and name in the top bar next to Activities showed up fine. The indication icon showed up fine. It did not show up in the Dock.
Based on a suggestion from the thread I linked at the top, I closed Slack via the Indication icon, and then restarted it. About 12 seconds into loading, the icon came up in the Dock.
Not sure how that would be fixed but… just an FYI.