I'm used to the Fenix default of holding the back button to return to the main screen, so I set up that setting on my new 965. On the Fenix, I can go the the ready page of an activity (e.g. press Run, wait for the gps, etc.) and hold the back button to both return home and cancel the activity. Doing so changes the battery estimate from some amount of hours (GPS battery) to days, so I'm confident GPS is off in those cases.
On my new 965, holding the back button on the run (or any activity) ready page seemingly doesn't turn off GPS (battery still shows ~20 hours, in my case). Pressing the start button from the home screen again to return to the run ready page and then pressing the back button twice (normal presses) also doesn't return the battery to its normal non-GPS setting. I figured out that the only way to exit the initially started activity is to repeatedly press the back button, which will cycle back into the started activity, back to the main activity page, and then to the home screen.
It also seems like I can create a stack of multiple pending activities by repeatedly going to the activity start screen, long pressing back, and then pressing start again for another activity, long pressing back, etc. The only way to undo these queued activities is to manually press the back button a bunch of times to exit each of them (only then will the battery life estimate change from hours to days)