In recent years IBM (and other companies) have become enthusiastic users of Slack to communicate. I must admit that I am not so impressed with Slack (partly because I was involved in developing some alternatives that I consider better), but I am leaning to use it nevertheless.
A confusing thing about Slack is how the concept of Workspaces and Channels intersect. On the far left of the Slack client you see a list of icons showing the Workspaces that you have joined. When you click on a particular Workspace , you will see a list of Channels in the Workspace and when you click on a channel you see the associated message. This sounds very straightforward, but the confusing thing is that you can have lots of Shared Channels and these will appear in multiple workspaces.
Like most messaging systems, Slack allows you to keep track of the messages you have read. I frequently load up Slack and read unread messages in each of the workspaces. However, I notice that when I switch to a second workspace the messages in the shared channel which I marked as read in the first workspace are still showing up as unread.
Eventually I figured out how to solve this problem. You need to either select the menu item "View\Refresh" or else type ctrl-R each time you switch workspaces. The Slack screen goes blank for about 30 seconds and then comes back with the unread marks correctly applied. It is annoying to have to do this, hopefully Slack with fix this bug soon.