I posted about this in “Questions”, but I think it might be more appropriate in “Bugs”, since it appears to be for any plugin, not just mine.
When I have a plugin running in a Figma file and I attempt to create a new file via the “+” tab, then go back to the original file, the plugin UI window has disappeared and cannot be re-launched until I close that file. This applies to any plugin that has a window open, not just the one I’m currently working on. It looks like Figma still thinks the UI window is open, but it’s not accessible to the user.
Original question post: Plugin window disappears when new tab is opened