Hello DavidD,
I tested this scenario on the stock invoice screen and I can not see any issue with the Due Date.
Here are the steps I took to test your query and the results produced;
1 - Created an invoice, entered Date 7/1 and Terms Net30. Result = Due Date auto updated to 7/31.
2 - Edited invoice, before saving, by changing Date to 8/15. Result = Due Date auto updated to 9/14.
3 - Edited invoice, after saving, by changing Date to 8/15. Result = Due Date auto updated to 9/14.
Are you running a custom invoice screen? If you are, I recommend that you test the stock invoice screen to see if the same issue occurs. If the same issue does not occur on the stock screen then we know it is an issue with your custom screen configuration. You can then replicate the actions from the stock screen and apply them to your custom screen. to resolve the issue.
I hope this helps. Let me know!
Noel