@julian I just updated to 1.17.2 and still have this issue where any topics created before the 1.17.0 upgrade don't display thumbnails.
Weird Digest Image Issue on Mobile
So, for some reason the default image for the digest email shows up weird on mobile (iOS / Gmail app). The "What's New?" image extends too far to the right side of the container.
Looks fine on desktop.
For reference, other notifications on mobile look great.