Following on Oli's discovery that the MX wind speed was zero for many hours Tue night/Wed morning, it appears that the MX anemometer sticks when the wind speed is below 5mph. The attached plot shows MX (red) and EX (blue) wind speed comparison.

I noticed the same thing when observing MX anemometer from the road side, it look a good gust of wind before it got moving. Tagging PEM.