To begin – this is a great question for our Nutrition forum! Rebecca will be able to answer with much greater authority on the nutrition end than I can. There are many factors which can influence retained body fat which go beyond training – mental/emotional stress, etc.
But from a training standpoint, my first thought is that you’ve become too efficient in the intensity zone in which you’re training. Basically – training works! By spending so much time at that low intensity you’ve engendered a metabolic change such that your body needs to expend less calories to do the same measure of work. Without adequate training stress, there’s no demand to change, i.e. let go of potential excess adipose tissue.
Since it sounds like you’ve done a good job of building a solid foundation of Z1/2 work with decent volume, consider increasing your moderate and high intensity load. This will increase the metabolic demand both in the workout itself but also post-workout in recovery.
I’ve found myself in a similar situation; through the base period i keep wondering why i haven’t trimmed down, but when i start adding the more specific, high intensity and muscular endurance work i gradually come into “fighting weight”.
Ultimately the goal is to find your body’s optimal composition – everyone is built differently so it is difficult and potentially problematic to try and apply some athletic “ideal” body composition to ourselves, though we often try! Focus on seeking your performance gains through smart training, and manage your nutrition and recovery needs in a healthy way, and your body will likely adapt in a way which suits you best.