Your "digital" headlight most likely varies the light intensity by sending voltage pulses to the LED. These pulses generate a bit of electromagnetic interference that is probably interfering with your HRM signal.

Aside from replacing the light with a simpler model, your best bet is to try to separate the two units as best you can. sometimes a few inches is all it takes. You could try a Minoura Space Bar to raise or lower one of the units or even try to mount the light head on your fork.