Well, speaking as someone who makes his living doing Accpac customizations, I strongly suspect a bug in that modded screen. Just because it doesn't touch that field doesn't mean it can't screw up occassionally.
One tricky part of OE screen mods is how to work with the OnRecordChanging and OnRecordChanged events. You use these events to detect things like whether the picking slip is printed, and adjust logic as required. It's quite possible that a flaw in this logic is kicking in and not updating the record correctly.
And FWIW, it should take the developer about 10 minutes to recompile his program for update 2. I know, because I had to do it for one of my clients recently.