You're repeatedly overwriting h before you do anything with it. It's not clear what you expect to happen. If you just want the union of masks, accumulate the union by generating the mask in the loop.
Something like this:
% preallocate the mask based on the appropriate page geometry
% use a variable name other than "image" for your image
% otherwise you're shadowing the function image().
mask = false(size(myimage,1:2));
% accumulate the union of masks
for k = 1:size(centroids,1)
ROI = drawcircle("Center",[centroids(k,1),centroids(k,2)],"Radius",10,'Color','r');
mask = mask | createMask(ROI);
end
imshow(mask)
Otherwise, you'll have to do something different.
