'Define canvas' doesn't work with images that need padding left and right

I've set up the 'define canvas' action thus:

> Define canvas 500x500 (center, center) #ffffff under image

When I upload an image that's too wide, eg 800x500, then I get white space added to the top and bottom of it, as expected.

However, when I upload an image that's too tall, eg 600x900, then the image gets cropped. What I would expect to happen is to get white space on the left and right of the image.

I've set up all the options to be symmetric so surely it should work the same way in each dimension.