Welcome to the forum!
What is the build number of your firmware? (Settings - About - Build Number)
Just want to check to see if you have the latest or not.
I will test this out and see how mine acts. I have not used Polaris in quite a while.
EDIT: Opened 3 different documents from my Gmail in Polaris. Left them sit for a few minutes each (2-3) and none of them closed automatically.
May also want to try a hard reset. Turn off and unplug your TF. Hold the volume down and power button for about 12 seconds until some text appears in the upper left hand corner. Wait another 10 seconds without touching anything (icon on the right should be highlighted in green) and it will then boot the OS. No data will be wiped.