Rather than a test at a higher HR, try a couple of tests at lower HRs first. I’m wondering if you’ve been testing near your anaerobic threshold rather than your aerobic.
* At the aerobic threshold, HR could rise but doesn’t.
* At the anaerobic threshold, HR doesn’t rise because it can’t.
Take the HR from your most recent test and multiply it by 80%. Then do a drift test at that HR. Be aware that it will feel super easy. That doesn’t mean it’s wrong.
Let the drift at 80% inform future tests. If it’s more than 5%, try something slower. If it’s less than 5%, try something faster.