I set the alarm on my 945 for 4:40 this morning and was up then. I got on my trainer and started a indoor cycling activity at 5:02. When I looked at the GC app on my Android phone, it showed the wake-up time as 4:42 (as expected).
Later in the day, I looked and the GC app showed that I slept until 6:20, which is about when I put my watch back on after completing my indoor cycling activity (I take the watch off while I'm cycling).
I'm on 9.00. I did not have this on the beta 8.78, nor any previous versions of the firmware.