I have the same issue.
Climbed to fl230, waited until PFD showed ALT. Changed altitude to fl030 and selected Vnav, the aircraft descends right away?
Vnav works perfect on Longitude, Caravan, TBM ect
Import from simbrief but most of the altitude constraints are missing except for the approach. I’m currently flying and I know my TOD point so I’ll activate Vnav then and see what happens
Also engines won’t start on occasions and I need to reset the flight!