Funny you should post about this, i had 3x15min this morning and had similar experience.
Mine were 3x15min @ 91% w/5min rest between.
Interval 1: AvgHR154, Max 163, Pw:HR 7.72%, EF 1.69
Interval 2: AvgHR162, Max 168, Pw:HR 6.37%, EF 1.60
Interval 3: AvgHR 164 Max 170, Pw:HR 6.12%, EF 1.58
The weird part was RPE didn’t increase between sets, despite HR clearly climbing between them. I think in-ride fuelling had a lot to do with that as I basically took in 90g of carbs and ~350cal in the hour that the intervals took place. Did the remainder of the ride for 45min @z2 endurance pace on just water.