Here's the problem. The effect of state law expungement of a criminal conviction on a federal issue is an incredibly complex one, with zero in reliable guidance. Add to the mix that, practically speaking, expungement affects records, not reality. Expungement doesn't mean it didn't happen. If the local paper reported it, or friend who knew talked about it on Facebook, or appears in some pre-expungement database, the information is out there and can be found.
The FAA's current viewpoint is pretty clearly reflected in the
MedXpress User Guide:
If the record of a conviction has been expunged, state the date that the record was expunged and the court that ordered the expunction.
There have been very few cases (falsification on the medical app) which made it to the NTSB and the language is all over the place. Some suggest they might be bound by the state expunction. Others say they might not. I say "might" because the cases didn't face the issue squarely - in all of them I've read there wasn't a real expunction. At best the applicant thought the record had been expunged, but it wasn't.
A further twist is state expunction law itself. Expunction in some is not an absolute. IOW, an expunged record in some states is still available for specific purposes. In some related to later offenses; in others, perhaps ironically, to later request for other expunction. The answer to the federal question may turn out to be different for different states.
I'm interested in the issue beyond its FAA ramifications, so I'm fairly familiar with the arguments on both sides. Heard some very heated ones - between lawyers.
No specific advice here. Just a very general one one - you don't want a medical issue to become a legal one. Before even getting to the FAA legal question of whether to disclose, there is what I generally see as the more important FAA medical question - the consequences of disclosure.
To the OP - I see that's what you asked at the very beginning. Hope you get a good answer to that to discuss with your attorney to make a reasoned decision how to approach the application.