This may not be a bug with DatePicker, but rather with the semantics compiler.

Reproduction:

With semantics enabled, open a DatePicker and dump the tree.

You'll see several SemanticsNodes for the weekday header (each one is actually an ExcludeSemantics), and for each empty container for days of the week that are part of the previous month.

The tree should not include these nodes - they do not effect anything meaningful and make for extra time in tree operations.