A glow always filled the shape's whole silhouette, so a frame with no fill
(a rounded-rectangle border with a soft glow, common in corporate decks)
came out as a solid tinted panel over the slide. PowerPoint glows such a
shape along its outline only and leaves the inside clear; a filled shape's
glow is unchanged.
Checked against PowerPoint's own PDF export of 31 real decks (597 slides):
the affected slides' mean pixel difference drops from ~29 to ~4 (of 255).