If MFP says you should be losing and you are not the most likely causes are under reporting of your calories consumed, over reporting of calories expended, poor data that you use for caloric and nutrient food content, and over stating your activity level for the baseline.

Beyond that I do not know what "ALC" is or entails. yes you can ride too much or over train but I really think neither of those will lead to weight stagnation. Weigh or measure everything and use good data on MFP. Set your profile to accuratly reflect your activity level without exercise. Use a HRM to plug in your exercise numbers or if you don't have one only eat back half or less than MFP says you burned. Unless you are burned out on riding just take a day or two off after your event. then follow that with some easy rides.

I don't know if my success will help you but what is working for me is to ride lots so that I can eat. The more I ride the more I can eat and I like to eat. most days I am close to my calorie goal +/- 100c it seems. This is with the built in 1000c / day deficit for 2 pounds a week loss. it is working. That said I think you know all this since you have lost far more than I have while using MFP.