1400 to 1800 caleries a day seems a bit light for someone riding a lot. Chances are you are losing muscle mass and lowering your metabolism. Also, the composition of your diet is important. If your diet is high in sugar or carbs you may be triggering a fat storage response. The other thing that might happen is you maybe be gaining muscle mass while losing fat but showing a net gain on the scale. This seems unlikely though because it might be hard to gain muscle if you are not doing resistance training and getting enough protein . Are your measurements getting smaller, particularly in the waist or is your waist getting larger? So many possibilities here and you have not told us everything. Your age is important as well.

When I started cycling back in my youth (33) the weight fell off of me and I ate everything in sight. I did also do a lot of resistance training and gained a lot of muscle (which helps in losing fat). At 70 it is much harder now.