My Garmins are the 800 version so I'm not a great source of wisdom on this subject. However, as is traditional on BF, I'll reply anyway. My guess is that the device thinks you're spending too much time above the aerobic zone it thinks is important. However your riding has obviously been productive of PRs. But maybe PRs aren't all there is, eh? There's also putting in 10 hours a week in zone 2 which is very productive of future successes. All zone 5, not so much. So try backing it off for a couple weeks and see what the device says. Which doesn't preclude one all-out hill effort per week, say, or a couple 20' sweet spot intervals. Yeah, try more structure.
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Results matter