I have a form that was in use for 6+ months now without issue. I updated to v3, made a small change and republished. The form then suddenly starting stopping submissions giving a error that a constraint was being broken, on a field that was not even relevant at the time and the user cannot even see. I had to add "" empty string now to the constraint to get around it. Always worked as designed before.
Is this a new bug? Why would a constraint be checked on a non-relevant field?