Long time later, I know… But I stumbled upon the same issue. Following your guideline is NOT solving it. PlatformIO keeps on using the first project in the workspace, despite expanding or collapsing all the others…
The only way out I see for the moment is to close the workspace (or folder, if you have only one folder open) and then select to open an existing project in PlatformIO and select the one you’re interested in.
Anything I’m missing?