Bug when zooming imagesc?

I am plotting an image and overlaying it with a semi-transparent mask. The mask labels various objects in the image and will be used for image segmentation. I plot both the image and the mask with imagesc. When I zoom, the alignment of mask and image shifts. I can't think of any reason why that should happen, so my suspicion is that there is a bug somewhere in the zooming of image objects, or in imagesc.

I noticed that the shifting only happens when one of the image dimensions gets large (over about 1000 pixels width or height).

Here's an example so you can verify for yourself:

N=5;
z = zeros(20,1200);
x =  z;
x([N,end-N+1],:) = 1;
x(:,[N,end-N+1]) = 1;
y(:,:,3)=x;
% x(:,:,3) = z;

figure
imagesc(y)
hold all
% keyboard
ih1 = imagesc(x);
set(gca, 'colormap', [1 1 1; 1 0 0]);
set(gca, 'clim', [0 1]);
ih1.AlphaData = 0.25;