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.
That catches all dom0 windows and assigns them to workspace 5.
I’d recommend reading this article and then use the debug print function along with --debug to understand how to identify the panel and exclude it from the rule(s).