In Template:Item Infobox and Template:Mob Infobox and Template:Structure Infobox the field "description" disappears when the image is a gallery. It's also not displayed on mobile.
Problems come from the fact that the "description" field is displayed as the label of the infobox image.
A solution could be to display the field description as an independent line. (<data source="description"></data>
). Then use <format></format>
and .css to make it look good (centred, with "", grey ?).
Does that sound good?