This is related to the relatively recent ability to set a default view besides
the All view.

If the new default view does not have a description, the "Add description" link
is available as expected. However, when clicked on, the text box which appears
is pre-populated with the description from the All view (assuming that the All
view had a description set). If this text is changed and saved, the
description for the All view is changed, not the description for the original
view.