Bangle.js 2 backlight randomly turns on.

The accel increase due to HRM can be anywhere between 1.5x -> 15x . Thats why increasing threshold does not eliminate it completely but you can increase the threshold further ::

//bit is changed to update settings
var c = Bangle.accelRd(0x18); // CNTL1
Bangle.accelWr(0x18, c &~0x80);
//adjust threshold, where 0x25 is the threshold value (increase/decrease)
Bangle.accelWr(0x27, 0x25);
//restore
Bangle.accelWr(0x18, c);

0x25 is hex, so 0x20 is lower, 0x30 is higher