I've switched off this option in DO13
Nevertheless, the folder tree automatically expands and focuses on any of the folders I select in any open lister.
Isn't this the correct option?
That option's more for improving performance rather than stopping the tree from changing. It controls how the tree auto-expands branches the first time you navigate into them. If a branch has already been expanded, the tree already knows the contents and so that option won't stop it expanding that branch a second time.
If you want to stop the tree from auto-expanding at all, you can use the lock button (which you need to enable from the Appearance page.)
More details here: Expand / Collapse [Directory Opus Manual]