Subplot panel height not consistent

There doesn't seem to be a way to set the total figure size for subplot().
Individual panel height can be specified in the call to plot_ly and (as i understand it) that height is scaled with the number of panels to become the total figure size.

I tried using this by passing to each subplot the individual plotheight multiplied with the number of subplots (nrows) so the total height of the total figure would be plotheight * nrows.
It turns out the heights aren't strictly adhered too.
Comparing the total height of the first two panels you can clearly see the x axes are at different heights:

generate_subplot <- function(nrows, margin = NULL){

  if(is.null(margin)) margin = c(0,0,0,0)

  p <- plotly::plot_ly(mtcars, 
    x = ~cyl, y = ~mpg, 
    color = ~rownames(.data), 
    type = "bar",
    height = 150 * nrows
  ) %>% plotly::layout(
    showlegend = FALSE,
    shapes = list(
      list(
        type = 'rect',
        fillcolor = "gray", 
        line = list(color = "gray"), 
        opacity = 0.05,
        x0 = 0, x1 = 1,
        y0 = 0, y1 = 1,
        xref = 'paper',
        yref = 'paper'
      )
    )
  )
  
  plotly::subplot(
    rep_len(list(p), nrows),
    nrows = nrows,
    margin = margin
  )

}

generate_subplot(2)
generate_subplot(4)
generate_subplot(8)

Start by running the supplied generate_subplot example for 2, 4, and 8 rows and compare the panel and x-axis heights in the rendered figures. Trace how subplot() combines the repeated plot_ly objects and their height values; done means each panel has the requested consistent height and the total figure height scales as expected.