The main issue is that the custom placeholder, defined in the Change placeholder field of a Date Picker, is not taken into account in the form.
The issue has been reproduced as below:
"Change placeholder" field is not designed to take patterns that change date format, the field is only to change the text inside date field if it is empty.
The accepted date formats are:
YYYY/MM/DD
YY/MM/DD
YYYY-MM-DD
YY-MM-DD
and they are accepted regardless of "Change placeholder" field.
Unfortunately, there is no way to specify date format for now.
This bug, https://jira.jahia.org/browse/FORM-902, will be converted into improvement (feature request) and will be implemented in next version of Forms.