Two-Channel PWM LED Driving at 12A Load Issue (Signals affect each other)
Voltage seems to jump up at the time when both channels are off -- so your power supply has poor load regulation or you have too much wires resistance that connect power supply with your circuit.
Voltage spikes are generally not dangerous here and caused by fast switching off of M1 and M2 transistors. To mitigate, you can add 100..1k resistor between M1/M2 gate and driver circuit (that is, drain of M4/M6).