Issue:
When the flight crew utilizes DBUS, the backup Mach target displayed on the right side of the speed tape is erroneous, whereas the Mach number displayed below the speed tape remains reliable.
Solution:
When DBUS is used, do not use Mach target mode (Ref: SPD/MACH selection on FCU). Speed target mode must be used irrespective of altitude and Mach number displayed below the speed tape can be used as secondary reference.