I’ve noticed the same thing. When my fitness is poor (end of a season or after an extended break from cycling) I notice I achieve high heart rates more readily (corresponds nicely with CX season). However, as I progress through training or regular riding my legs ability to drive a high heart rate fades. This leads me to believe the limiter is a muscular endurance, the heart and lungs are willing and able but the legs just don’t have it in them.
A buddy advises me to focus on higher cadence pedaling. This might be it, on my last few ramp tests I have mostly focused on high cadences and achieved high HRs but on my last ramp test I did it with lower cadence and finished with a significantly reduced HR but a higher result and legs just as fried as they normally would feel.
Over the years I’ve seen the pattern repeat, start training with high HRs and then as the training goes on it becomes harder and harder to achieve those same HRs (even at increased efforts). I’ve seen this on TR, I’ve seen in spin classes, and I’ve observed this with my Strava PRs too.