This is an old one but I'm starting to notice it again. Every time I'm about to get to my apoapsis the vertical speed flickers between a value under 10 and the same value +10, or a value over -10 and the same value -10 (6 and 16 or -4 and -14 for example)
Another repro case that is about 1 minute from repro:
https://www.simplerockets.com/s/vN30xZ/Vertical-Velocity-Jumps
Sometimes. Not always. Maybe give it one or two more orbits and keep checking its vertical velocity at the periapsis.