Please see the app note below about the WHRM algorithm performance. The app note shows that the algorithm was only validated against humans with BPM not going below 60BPM. E.G. here is a graph to compare the algorithm output against a reference device (ECG Chest Strap), and two error bands. It only goes below 60BPM once, to 55BPM.
Please note that this health reference platform - the MAXREFDES105, has not been designed to meet any medical standards out of the box. Please see the ADI Reference Designs Disclaimer. Reference Designs Disclaimer | Analog Devices