Magnify all maps broken

Broken with or without 'optimise raster map resolution'- behaviour is identical either way.

Firstly, the preview of the map is zoomed in far more than would be expected from any selected zoom.

Viewing a raster map (it's easier to see the problem on a raster) at zoom level 15, changing the zoom all maps setting to 125% has no effect.

Changing the zoom all maps setting to 150% makes it select a new map scale- level 14 (even though the preview is still of level 15), and zoom it to 200%.

I expected the zoom all maps setting to have no effect on the map scale selected, but to magnify any given map by that % (like a magnifying glass on a paper map of your choice), not to select an equivelant map scale (like showing a different paper map of the same area, that doesn't contain the detail necessary, under another, different magnifying glass).

Cheers