I've had my 945 for a week or so, moving up from a VA HR and Edge 520 - the 945 replaces both very nicely and opens up lots of other options for me.
Today I fitted and tried the Quick Release (quarter turn) mount on my bike and I was surprised to see the 945 failed to auto switch to my HRM strap after I removed it from my wrist to fit on the bike. The strap only seemed to be used if I clicked to select an activity and then, going back to the main watch it was used but dropped after a short time, showing "--" for HR.
Edit: I should note the HR strap showed "Connected" in the sensor list whenever I checked.
My VA HR certainly used to switch from what I recall and stay using the strap until it shutdown after removing it.
Is this a bug or a feature?