People List view - date sorting results in empty dates at top & bottom w/non-empty dates in the middle. Why?

No. There 4 Quarter dates:
Mar Q - 1st quarter
Jun Q - 2nd
Sep Q - 3rd
Dec Q - 4th

I’m familiar with them both from earlier versions so it’s an oversight that Quarter dates are not covered in Help. They are commonly used in recording BMD from UK GRO Indexes. See