OK, so my workaround is as follows.

For each track:
- solo the track in the mixer
- hit Generate and Play (ensure the track works)
- now drag just that track to the drop area to generate aiff
- when done, drop into external program

It's quite possible this could be pilot error--this is only my 2nd day with the program. Although I have many years of experience with all manner of music software on both Mac and PC, and I'm a software engineer by trade.

Integration with external programs really elevates the usefulness of this tool, so I hope this is fixed soon.

Thanks,
-Tony