if display altitude doesn’t match with the actual alt, for example display 8000, real alt 6500, if you press VS the plane (Autopilot) will enter in a default -500 fpm. If the plane matches the display alt (8000) and the real alt (8000), if you press vs button the display will change to 0 fpm and now you can adjust it fine for climb or descent. The vs button is bugged. My recommendation is not no press it if you are not matching the alt in the display with the real one. Use in alt mode when the plane is stabilized and matching the desired alt with the real one. And it’s doesn’t matter what vs speed do you set before pressing VS. if you are not matching the altitude it will engage -500 by default
I’m not at home now. I will report more details later