If you have HRM set to always on then https://github.com/espruino/Espruino/issues/2141 could be your issue.
@myownself started
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
If you have HRM set to always on then https://github.com/espruino/Espruino/issues/2141 could be your issue.