I added a command chip to the top of my first stage and had it report its vertical speed. Once it seperates from the capsule the vertical speed it reports is still the capsule data and not the falling first stage. This may be a feature as a IRL command chip will not include flight data sensors and a sensor pack would be a new part one would need for craft position, attitude, and velocity data.
I built a new craft to test this and it all behaved as expected. Once the core stage was dropped it's Vert V began reducing as the command capsule flew on with different Vert V value.