Very often, working on a complex appearance, we accidentaly loose the object selection, and the stack we edit remains unassigned.

The panel then gets 'No selection' in the top row... but if the attributes stack is long, this line can get hidden if you scroll the panel down.
Then, if you select something else, all your changes get lost.

The workaround now is to be vigilant (ha) and save your appearance as a temp style before you select back the art you wanted to edit appearance for in the first place.
A better solution would be to make the 'No selection' row fixed (or maybe make it fixed always, not only when nothing is selected, to make it consistent).