DIAGNOSIS That's normal and by design. It's a cooling-down period for the magnetron (the part that generates the microwaves) and other components. The fan continues to run to dissipate residual heat after the cooking cycle ends.
FIX There's nothing to fix; this is intended operation. You cannot disable or shorten this cool-down cycle on this model. It's a protective feature to extend the life of the electronics.
IF THAT DIDN'T WORK If the fan runs for an excessively long time (over 10 minutes) or never turns off, then there might be a faulty temperature sensor on the magnetron or main board causing it to over-cool. That's a more serious fault.
HEADS UP Yeah, this one's common and surprises people. It's not a defect. Just let it do its thing. Unplugging it to stop the fan isn't recommended, as it cuts the cooling short while components are still hot.