I’ve reproduced exactly the scenario you describe in AxCrypt 2.1.1509.0 running Windows 1607 (14393.1358):
AxCrypt signs out when Windows sends the idle timeout signal
It’s not a bug in either AxCrypt or Windows; this is how Windows notifies other processes/applications that the system is at rest.
I set the “Display > Turn off display after” to 1 minute whilst AxCrypt was signed in. After the display switched off and I moved the mouse to wake up the system AxCrypt showed as signed out.
You originally said “When it was first installed, I only needed to enter the password once per Windows session”. I even ran AxCrypt 2.1.1494.0 and there was no difference to this behaviour. I can only assume that you’ve either changed the Display option or that during the Creators Update Microsoft have changed your personal settings (they do this all the time!)
As AxCrypt signing out whenever the computer is put to sleep (sleep, hibernate, screensaver, display off), locked, logged off, restarted or shutdown this is an intended feature. The only thing you an do is: change the option to an acceptable number of minutes or switch it off altogether.