[tzs]

The idea is that stuff dislodged by the flossing gets brushed away and spit out when you brush. If you floss after brushing, it is more likely the floss debris will remain on your teeth. You’ve just moved it from between to a side.

That was the argument I found when I recently looked into this. It makes sense if all you’re doing is brushing and flossing. If there are further steps in your routine that would take care of the flossing leftovers, I expect the order does not matter.