That happened to me too. Basically I had some lua code in my devilspie2 config to assign windows to workspaces. One day I had the genius idea that I wanted all windows that I hadn’t otherwise assigned to default to a specific workspace.
Long story short: my code ended up assigning the panel (which is a window) to a specific workspace. I bet that’s what’s happening for you too.
If you don’t see it, post your devilspie2 lua code.