You can help isolate the issue by pausing dropbox syncing. Quit dropbox entirely and see if the issue can be reproduced. If the problem still happens without Dropbox, we should look at other causes. I asked about Dropbox because that’s the only other case I’ve heard of this error, and for that user, pausing syncing made the issue go away. It does seem like a bug, but if it’s caused by Dropbox, I won’t be able to help solve it on this side.
We read both timestamps in OmniGraffle Pro when you open the document so that you can use our variables for either inside the document for timestamps. See Insert Variables under the Edit menu for the list of variables. To the best of my limited knowledge, OmniGraffle is a standard macOS document based application and follows the Apple Developer guidelines for Sandboxed applications. OmniGraffle documents have been around longer than I’ve been programming, so I apologize if I didn’t answer your question.