I am using Slack on a Macbook Pro with an external monitor. For some reason, the slack window is on every space on a certain screen. i.e. if I have a slack window open in the center of the screen, then even when I move one space right or left, the slack window will retain its position and will be visible on every space.
I can't share the video because of the sensitive content but If the question isn't clear, then I can try to hack something up to make a screencast.