I have seen this, too.
It does not need submit, it happens while editing an issue.

To reproduce:
- open an issue with a date set to 2016-03-31 (oops, long overdue)
- to get in the correct range, change the 6 to 7: 2017-03-31, but do NOT
submit yet
- now click on (cal) to get the popup to adjust it to a suitable date
Result: The calendar popup shows March 2016
Expected: The calendar popup shows March 2017

Or if you clear the date, February 2017 should show, but it still shows
March 2016.
When fixing this: Beware about invalid dates, as there was no error
checking at this point.