Is ofImage::mirror(bool, bool) supposed to make a weird symmetrical thing instead of flip?

I was expecting/wanting mirror() to simply flip the image over the indicated axes.

Instead, I am seeing it do a weird effect where the image isn’t just flipped, but turned into some sort of symmetricalized version of itself.

So I have an image like this:

that during setup I load and set up like this:

Image.mirror(false, true);

And instead of a simple flip, it ends up transmogrified into this (ignore the rotation and green):
Clipboard

Is this a bug in .mirror(), or have I just not found some bug in my own code?

Or is it supposed to do this, and I need to achieve a simple flip in some other (?) way?