There are 3 ways to launch apps-contacts-OkGoogle pages (what is official name of pages?)
Please don't use swipe-to-the-left from right edge for lauching them.
Just use a tap or say Ok-Google.
If you use swipe, your underlying clock is still working underneath.
Or communicating with companion app using Data API
Or just simple drawing digital fonts
Or various weather/battery/calendar/fittness things
And some apps like Cardiograph may cause silent working of your clock. I dont' know, You dont know, even Google dont know what app will do.
My simple guess is Cardiograph app seems to use OpenGL?
Google unofficially said its not a bug. it may be a feature!
It may be not a big deal, but waste is a waste.
Until this will be fixed, as possible as you can, please use TAP or OK-GOOGLE, not SWIPE.