Correct, you can’t open a project file in Studio 2 if Studio 2 was opened as a plug in from a host editor, but you can save a project file in that mode.
Having to subsequently open Studio 2 in freestanding mode to be able to open the project file may not be optimum, but it may save you having to redo complex masking. After making what ever changes are necessary, export the image from Studio 2 as TIF/TIFF and then open that in Photoshop or Affinity or whatever host editor you use. You may have to redo some of the previous post Studio 2 editing, but that is possibly less finicky than having to redo all of the masking from scratch.
Studio 1 does allow you to both save and open project files when used as a plug in, (at least that is the case when it is used as a plugin from Affinity Photo. I don’t use Photoshop any more so I can’t confirm it’s the same for that). When you open the project file it becomes the active image in the image pane/panel and after any changes you make is returned to the host editor when you click on OK.
Hopefully at some stage that functionality can be added to Studio 2