OHR automatically off when chest strap is worn?

probably a lame question, but i decided to start wearing a chest strap on certain runs when i was really trying to closely monitor my HR. the OHR, IMHO, has been far superior to that on my 935, but it still has occasional cadence locks where my HR will go way up for moments during a run.

anyway, the watch correctly detects my chest strap and appears to be using it (much steadier data and no pulses of cadence lock), but as far a i can tell, it seems that the OHR is still running as the green light is still on unless i take the watch off. i have the OHR set to "auto" which i would assume should shut the OHR when i'm using  chest strap.

anyone have a similar experience or insights to share on how to make sure the OHR is off when using a chest strap? or is this a bug?