DIAGNOSIS Yes. The microwave has a small internal backup battery (typically a coin cell) that maintains the clock and settings during brief power outages. It's depleted.
FIX This model doesn't have a user-replaceable clock battery. You'll need to live with resetting the clock after a power loss, or contact Breville support for a service option.
IF THAT DIDN'T WORK If the clock resets even when the unit is plugged in and appears to have power, it indicates a deeper issue with the main control board, not just the backup battery.
HEADS UP The clock resetting doesn't affect cooking performance. It's purely a convenience feature. Constantly unplugging and replugging to 'fix' it can stress the power components.