Great that you decided to preserve this test! I could make for a nice accent piece somewhere.
One thing I've found beneficial with working upside down is the glue will make the top surface flat, regardless of varying thicknesses of veneer. Fortunately I've not burned through any soft/thin veneer, I'll do a light pass over with 220 on a ROS, thenĀ maybe 320 if I'm going for something special.
I'm also surprised on you getting some warpage, I have gobs of backer I use for big stuff but maybe I never looked close enough at the small panels to notice?
Anyway, looking great!