Hi Mark,
Thank you for using Syncfusion products.
We have checked the reported issue by placing SfDatePicker at bottom and prepared a sample to move the Popup containing SfDateSelector up, when BottomAppBar exists. In the sample, we have set top Margin for the popup used in SfDatePicker so that BottomAppBar will not hide it. You can download the sample for the same from the following link:
Sample:
DatePicker_AppBarRegards,
Jessie