Today was the first time that I reached a low battery fail safe level before it reached my user set warning level (which I have at 15%) because I had walked with the craft about 20 meteres from my home point. The manual states that when the fail safe level is reached "The aircraft will automatically return to the Home Point if no action is taken after a ten-second countdown." However, I did not get 10 seconds to land on my own: My controller gave a single beep and the the craft began to ascend. This is concerning because I was under a canopy of trees at that moment.

Any other Mavic owners notice this?

Same thing happened to me.

hmmm... I wonder if you can get that info in the logs? (Log to show whether the count down happened or not)

Could you please sync the flight record from DJI GO app and provide me with your DJI account email?
Also please upload the flight date to dropbox and send me the link.
You can use the same method as you would for the Phantom 4.
Here is a video tutorial for reference: