Apple Watch 4 - Not counting Flights of stairs during "Indoor Walk"

When I walk up and down 4 flights of stairs while not in any workout mode, the watch activity records that I've climbed 4 flights.

When I walk up and down the same 4 flights of stairs while in "Outdoor Walk" workout mode, the watch activity records that I've climbed 4 flights.

When I walk up and down the same 4 flights of stairs while in "Indoor Walk" workout mode, the watch activity doesn't record that I've climbed any flights. Why not?