I figured this out, at least for my situation.  Hopefully it will help others, too.

Our form loads with a Token in the url, used for lookup authentication.  We have a home grown service that builds the tokens and returns the url with the unique token in the url.  But we have users copying the link, with the token included, and sending it to other users.  The token doesn't belong to them so it causes problems with their form.

To prevent that, I strip the parameters from the url like this...

window.history.replaceState({}, document.title, window.location.pathname);

Apparently, there is something happening behind the scenes, possibly with the pdf generator, that this messes up.  I commented out this line and the problem goes away and the form works perfectly.  Uncomment it and problem returns. 

The next question was why did it work for my test form but it didn't work for this form?  I believe it is because the test form is so small and has no javascript that it completely renders the form before the pdf generator can act on it.  But the form that was having problems is much larger and has extensive javascript.  I think it takes so long to render the large form, that the javascript is still executing when the url changes and the pdf generator has a problem with that.  That's my theory.  Whether that is correct or not, removing that command fixed it.

So why did it just start being a problem?  Our servers patch weekly...sometimes more often.  And there likely could have been a patch that changed that somehow.  

Hope this helps...