I’m kind of late to the party, but I’ve been riding a fixed gear on Zwift since 2017 using a Stages power meter (DA track crank) and Elite smart trainers, including the last month on a Tuo. I agree with Aoi, who has done an excellent job in explaining the parameters and the likely causes of the problem.
I would add that the wattage Zwift shows onscreen when you are descending at 4+w/kg should be over 300 watts. If not, you need to check the weight you’ve logged in for metric units is around 73kg and not significantly lower. Also, make sure if you’ve selected imperial units as the measurement that you input 162 lbs as your weight. (If you input 73 lbs, this is probably the source of your problems.)
Otherwise, it’s not that unusual when you are riding at the back of a group of heavier riders for this problem to happen. I’m 150 lbs or 68kg and I sometimes have to put out 3-4w/kg just to hang on the back of a big group on a long descent. To avoid this I just make sure I’m near the front half of the pack before the descent starts and put out the effort to stay there when it starts.
You may have another problem which isn’t the cause of this problem since Zwift is using the wattage reported by the paired power meter to calculate your speed. However, the wattage is probably inaccurate if you are not using a separate power meter like a Stages crank or Garmin Rally pedals as your power source. If you are using the M2 as your power meter, you need to do a roll down or spin down using the Saris app after a 10-15 minute warm up. This is almost impossible to do on a fixed gear bike because it requires you to pedal up to a speed over 30kph and then stop pedaling and let the rear wheel coast to a stop on its own. Since you have no freewheel and are on a fixed gear and not a singlespeed, the only way you might be able to correctly do this (and avoid injury) is to use platform pedals where you could just lift both feet off to start the spin down, and try to maintain your balance on the saddle without them.
I suspect your uncalibrated M2 is reporting higher wattage than a properly calibrated power meter would. Keep in mind that you can’t just use a single speed or freewheel/derailleur bike to calibrate the M2 and then mount your fixed gear in its place because it has to be the same tire that is used in warm up and the spin down procedure if the calibration is going to have any accuracy.