DIAGNOSIS The oven has a small backup battery on the main control board that's supposed to keep the clock and settings alive during brief power loss. That battery is dead or missing.
FIX You need to open the outer casing (unplug it first!) to access the control board. The battery is usually a standard 3V CR2032 coin cell in a holder. Swap it out for a fresh one. This is a known issue on earlier production runs.
IF THAT DIDN'T WORK If a new battery doesn't fix it, the circuit on the control board that charges/maintains that battery (a tiny capacitor or diode) could be faulty. In that case, the whole control board would need replacement to solve the clock issue.
HEADS UP While you have the casing open to replace the battery, be super careful not to disturb the wiring harnesses, especially the ones connecting to the door switch and temperature probe. A loose connection there can cause bigger problems.