I tried toggling the setting off and on again, and it made no difference.
If the Audio item in the tree is open when I close/restart MC, and then I open Video, Audio remains expanded. And if I leave Video open when I close it, and then switch to Audio, then Video remains expanded. And so on and so forth.
That is the behavior I get if that setting is UNCHECKED. That's the normal behavior for that state, as I understand it.
To switch to the "other" behavior, this is what works for me:
1. Put a check in the auto-expand box.
2. Collapse ALL items in the tree.
3. You could restart MC here, but for me it is not necessary.
Now:
4. Click on the word Audio, and it will expand.
5. Click on the word Video and it will expand and Audio will collapse.
6. Exit MC, and restart it
7. Video will still be expanded
8. Click on Audio, and it will expand and Video will collapse.
It's important to note, behavior is different if you click on the word Audio, than if you click on the disclosure triangle to the right. The above behavior was generated by clicking on the WORD itself. If you click on the disclosure triangle to expand it, that expansion will "stick" even when you click on a different section.