Why does raster plot "north" as "south"?

Question

I have no problems using the raster package when I stay within the confines of the provided functions but when I go off-piste and manually mess with the internals I get into trouble. In this example I am trying to change raster matrix element but is seems that "south" gets turned into "north" when when plotting? What is happening? Thanks.

    # test raster plotting
    library(raster)
    #> Loading required package: sp
    
    m1 <- matrix(1,nrow=10,ncol=10)
    
    r1 <- raster::raster(m1)
    r1 <- setExtent(r1,c(1,10,1,10))
    r1
    #> class      : RasterLayer 
    #> dimensions : 10, 10, 100  (nrow, ncol, ncell)
    #> resolution : 0.9, 0.9  (x, y)
    #> extent     : 1, 10, 1, 10  (xmin, xmax, ymin, ymax)
    #> crs        : NA 
    #> source     : memory
    #> names      : layer 
    #> values     : 1, 1  (min, max)
    # looks as we expect, all 1
    plot(r1)

    # set first, bottom, most southerly row, to zero
    m1[1,] = 0
    
    r2 <- raster::raster(m1)
    r2 <- setExtent(r2,extent(r1))
    r2
    #> class      : RasterLayer 
    #> dimensions : 10, 10, 100  (nrow, ncol, ncell)
    #> resolution : 0.9, 0.9  (x, y)
    #> extent     : 1, 10, 1, 10  (xmin, xmax, ymin, ymax)
    #> crs        : NA 
    #> source     : memory
    #> names      : layer 
    #> values     : 0, 1  (min, max)
    plot(r2)

    # top, most northerly row is displayed as zero
    Created on 2022-02-19 by the reprex package (v2.0.1)