setResizable(true, false) working for VST3, but not VST2/AU

I’m a bit stumped by this one.

In my AudioProcessorEditor constructor the last thing I do is:

    setResizable (true, false);
    setResizeLimits (GUI_WIDTH * scaleUI, GUI_HEIGHT * scaleUI, GUI_WIDTH * scaleUI, GUI_HEIGHT * scaleUI * 2);
    setSize (GUI_WIDTH * scaleUI, GUI_HEIGHT * scaleUI);

where

const int GUI_WIDTH = 1200;
const int GUI_HEIGHT = 800;
float scaleUI = 0.666f;  // this can be 0.5, 0.666 or 1.0

In Live I can resize the VST3, but VST2 and AU are not resizable. In GarageBand when hovering the mouse over the window borders it shows me the resize mouse cursor, but resizing does not work.

Is this just a limitation of VST2/AU or have I just overlooked something/done it wrong?

edit:
I added

    setResizable (true, false);
    setResizeLimits (500, 500, 600, 600);
    setSize (550, 550);

to a blank Plugin Project and confirmed the same behaviour, so I guess it’s a limitation for VST2/AU?

edit2: bump, can anyone confirm that this is a limitation, or if I just did something silly?