Trouble holding cadence in outdoor workouts in SS and Threshold workouts

With the nicer weather, I have been doing more TR workouts outdoors. Historically, I have focused on either short VO2 work, using hills matching the lengths of the intervals, or longer endurance rides. I have no problems working those outside. My normal outside cadence is 85 to low 90s.

However, I have now started sustained power build. I am finding it very difficult to keep the target watts for SS and Threshold work. I can hit and sustain power at 75-80 RPMs, but very hard to keep 85+.

Two factors that I think could be contributing - 1) FTP is at an all-time high, about 10% higher than my prior highs. I have been able to do most indoor workouts at this level, though hard. And at 90+ RPM (regularly 100+ when doing vo2). 2) I have a wide range 11-34 cassette and a 50 up front. I can do the lower cadence SS work on the 13 on the flats, but lowering to 15 (next cog) to improve cadence requires me to spin too fast.

I notice the increased leg fatigue of these lower cadence workouts. Am I training the right systems at such a low cadence? Should I expect to see an increase in cadence as I progress through training? This is my 4th season with TR and the first seeing this issue (but also my biggest FTP gain).