tint() on createGraphics() buffer in p5

I'm trying to change the opacity of an offscreen graphics buffer in p5, and can't figure it out.

consider:

var pg = createGraphics(...)
var image = loadImage(...)

tint(255, 50)
image(image, 0, 0) <----- this works
image(pg, 0, 0)    <----- but this doesn't

working example here

tint(255, x) should leave the coloring unchanged and set the opacity to x, but seems to have no effect. works fine on images, though... what am I missing?