Voltage drop from PCB ground plane to motor driver
Building on top of @Reroute's comment, this voltage offset is probably due to the non-zero collector-to-emitter voltage of your bipolar transistor. If the intention was to cut the power of the motor when it is not being used, you could have done it by setting the motor in Stand-by mode.

According to the datasheet, once in stand by mode, the internal circuitry is turned off and the current consumption drops from \$1.5mA\$ tp \$10\mu A\$. Setting the motor in stand-by mode requires only that you pull the pins \$F_{IN}\$ and \$R_{IN}\$ low.