My experience with HR while on Reta has come with a fair share of ups and downs.
The thing that worried me most was how high my max HR climbed during hard workouts. My farming Fenix was showing close to 220, even though I never actually felt my heart pounding that way. Once I switched to a chest strap, that strange reading disappeared, and I'm now right at the number I should be: 188.
5 months in, my RHR has been gradually easing back down. It used to sit around 48-53, then it climbed to 65-68. These days I've been steady on 4mg for roughly 3 months, and my RHR is at 62, with an expectation that it will fall under 60 as time goes on.