The date picker control has its specifics and these need to be taken into account when automating such.
Using the calendar to select the desired date may be applicable in some scenarios if using data driven approach. This is because the displayed month is different depending on the current date.
However, when you need to enter an exact date by each test execution, it will be best to enter this manually as text in the input field of the calendar control.
Please, give this a try and let me know if it works fine for the date picker you are automating. In case you face any troubles, please, let me know if you can share access to the application under test and demonstrate the control, or alternatively, you can prepare a test to show the issue against a demo page using the same control.
Thank you for your cooperation in advance.