Declining Cardiac drift?

So I did Cumberland this Sunday: https://www.trainerroad.com/career/thomaseh/rides/71730811-cumberland

Watching my heartrate it declines after 30 minutes, I have heard of cardiac drift where the heart rate goes up after 10-20-30 minutes. But not the other way round!

This is 80-85% of FTP and my Heartrate was first 30 minutes: 134/186 (72%) and last 30 minutes minus cooldown 130/186 (69%). I cant understand why my HR is so low with work at such an high percentage of FTP neither.

Ramp test 16. January: https://www.trainerroad.com/career/thomaseh/rides/70887892-ramp-test so my FTP should be correct.

Have anyone heard of this phenomenon before? And where can I read more?

There could be a problem with your HR monitor, especially if it’s a one-off.
Or you could’ve been getting tired.

It happens regularly: https://www.trainerroad.com/career/thomaseh/rides/69090855-aet-9-75-

If I get tired, the HR should be raised?

If you’re fatigued, your heart can’t do as much as often. You’ll struggle to reach your max heart rate.