Apple Watch 8 and fights of stairs climbed

I work in a place that has many kilometres of corridors and numerous staircases. One way I get exercise is to walk around the place on breaks - usually a kilometre or so at a time, with at least eight flights of stairs each time. If I walk using an “indoor walk” workout, my watch doesn’t count flights of stairs. If I walk without that, it counts flights of stairs. It took me a while to figure this out - at first I thought there was something wrong with the watch. This strikes me as a peculiar “feature”. Not sure what it’s purpose is.