Difficulty maintaining power at recommended cadence. Is recommended or preferred cadence better?
GCN did a couple of videos on the subject (1, 2) . In the first linked video, the presenter said that "what current research seems to point to is that is that people ride at their energetically optimum cadence when they're toward their maximum power output. However, they tend to ride at too high a cadence when they're riding at lower intensity." Also that everyone's different, leg size factors into cadence, etc.

Sounds to me like you should do what your body is telling you to do. You're obviously experienced enough that you're not going to blow your knees out riding stupid-low cadences.