Repro steps:

1) Create a new project and import the package I attached
2) Open scene Negative suspension force
3) Run and notice the telemetry data in the Game view
4) Press Enter to give force to the object
5) Notice that F(Which shows WheelHit.force data) becomes negative for a short time
6) Pressing R restarts the position and pressing T enters slowmotion so you can see it a little bit better
7) By changing Vehicle - Huge Damper -> WheelColliders child component Damper value to a lower will stop generating negative force