Document picker dialog does not open at configured folder if rootPath value is relative

If a manageContent tag is configured like this:

<@hst.manageContent templateQuery="new-banner-document" componentParameter="document" rootPath="banners"/>

...I expect the document picker to open with the folder "banners" open. It opens now at the root of the Content / Documents perspective.

The description of this parameter in the .tld file is "Path to the root folder of selectable document locations. Can be a relative path to the channel's root folder or an absolute path in the repository." If that is still valid...?