font sources only allow to specify advances, the sidebearings are computed. Makes more sense for the sparse metrics to have 0 for the sidebearings as the glyph that's meant to be 'sparse' is most likely empty with no outlines
as discussed in https://github.com/googlefonts/ufo2ft/issues/501
Three masters, Regular and Bold have glyphs .notdef, a, e; Medium only has .notdef (but empty, no contours) and e.