Hi Eric,
Did this start happening recently? If so, I'm guessing your hunch might be right for large libraries. If not, it was always happening, right?
On the other hand, the list/library column data is stored in a single SQL table in SharePoint and that table would also trigger workflows. The reusable workflow can only see the value if it's promoted as a column to the library so it should be seeing the correct value. You're not doing some Nintex XML lookup or something are you?
Keep in mind that property demotion (setting field data back from library) is inherently a bad practice because no XML validation happens and the forms could break (won't open again) without you knowing. Simple workaround would be to ask people to open the forms in the browser to change status.
Bottom line: this shouldn't be happening based on ShP architecture, but your form library is also way too large. Please consider archiving in separate libraries or moving it all to SQL.