If I exit PlatformIO and then re-open PlatformIO it opens with the same project open as before. However, when I change to a different workspace, and then return to the original workspace, the currently open project is different to the one that was open before. It would be more intuitive if the behaviour in both scenarios matched.
I’m afraid that what you are seeing is a problem in VSCode, not in PlatformIO.
VSCode has a few foibles like that.
Cheers,
Norm.