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I wear a Garmin Forerunner 645, which has a built-in optical HR sensor, just like your Gear S3 Frontier. Optical sensors are generally regarded as less accurate than chest straps (which is what I use when I'm running), but they should be close enough to evaluate general trends.
A resting heart rate of 97-124 is awfully high (mine is consistently in the low- to mid-40's - which is lower than average but typical for more athletic individuals - and that's regardless of device - Fitbit Charge 2, Apple Watch 2, Polar and Wahoo chest straps, and now the Garmin watch).
It's likely that you're either wearing the device too loosely (there ought to be as little light leaking into the sensor area as possible), or that there's something wrong with the HR sensor and/or associated software.
Is this spike in RHR new, or has it been like that since you started wearing the device?