Ive had that issue as well, and usually tapping the date again fixes it. I’ve had a few other fields on various parts of the app where I wasn’t able to “Save” after editing, and I figured out that the fix was to edit a separate field (could be just deleting and re-entering a word), and then the “Save” option would appear. A very clunky app interface that leaves a low confidence level that all the information that was entered is appropriately submitted.