Using the new feature I've noticed that after exporting audio using 'Selection' that sometime during working in the project it reverts back to 'Project'. I haven't caught it when it changes but I have an idea it has to do with the play/ transport.
 
Not necessarily a bug It'd be nice to keep the choice as previously selected / no confusion next time one exports expecting the same setting.

I caught it, it reverted back when I double clicked on an audio track.

I haven't actually used the Export module yet, but from what I can tell the Selection option is greyed out and the Project option enabled instead whenever there isn't an explicit selection made along the timeline.   Maybe that's the case here?
 

The selection option will revert to project if the current selection gets cleared. 
We did that intentionally because otherwise if you export it will give you an error that there is no audio selected.
 
We were trying to get away from the somewhat confusing paradigm of exporting the full project vs a selection based on whether anything was selected. Thats the way it works with the standard export dialog and new users are always confused by it. Therefore we decided to make the selection vs entire project choice very explicit and not implied just be selection.

Noel Borthwick [Cakewalk]
 
We were trying to get away from the somewhat confusing paradigm of exporting the full project vs a selection based on whether anything was selected. Thats the way it works with the standard export dialog and new users are always confused by it. Therefore we decided to make the selection vs entire project choice very explicit and not implied just be selection.

As someone who intends to export the entire project 99% of the time, I'm a big fan of this change.  Thanks!
 
AMB
 

Thanks Noel. 
Still I wish it could remain locked until I need to change it. I only use 'Project' at the very end when rendering down my source mix from stems. Otherwise while editing, mixing I only use 'Selection' to  render stems from selected tracks.
It's a great feature either way.

We've improved this for the next release so that the selection radio setting won't auto switch on you. Hopefully this will address your workflow issue.

Thanks Noel