I live at flat 8 of building 5, which through a strange quirk in the way addresses are created in my city & then recorded at the post office, accepts mail intended for both 8/5 and 5/8. A nearby neighbour in flat 5 of building 8 also receives mail intended for both 5/8 and 8/5. Most mail providers automatically adjust "flat 5 building 8, street name" to "5/8, street name", so we've got no way of fixing it.

Everyone in the estate who lives in flat 9 or lower in their respective buildings has the same issue, and the issue also occurs in a bunch of other new builds in the city.