I have the same issue. I need to display date in format: “M yy”, so I set Save format as “M yy” and Display format “dd/mm/yy” (default), but after update value isn’t displayed in input field. I can get this value in front-end and it is in right format, so everything works okay, except displaying saved value in backend.
I can see entered value after update only if Save format is “yymmdd”.
I was having the exact same issue. I got it to work for me by changing:
Save as timestamp to No
Get field as timestamp to Yes
I assume this may have something to do with strtotime but I am not a developer. The format I use is DD MM/d/yy for date and h:mm tt for time. Anyways, this worked for me so maybe it will work for you.