PWM motor control - weird voltage ringing issue
One possibility is that you placed MOSFETs too far from IC and you have parasitic inductivity. As the problem persist when adding gate resistance, it would mean that ringing is not an artifact of gate driver circuitry, rather a load problem.

Further you say, that when changing the motor with a different one, this problem vanishes. In my opinion, you have kind of capacitor mounted in between brushes, like EMI filter. With your series inductor this makes a LC circuit with its own resonance as you see on the scope.

EDIT:

Further, it seems some weird form of oscillations, like peaking generator. Could be that you series chokes are saturated?