Slightly different issue and alternate solution... I have had a similar issue with the canvas *apparently* not extending equally on all sides. I can't reliably reproduce the issue, but it most frequently occurs when I have extended the original image using generative fill and somehow cropped. (There is one image that exibited the issue without having been extended.) I say "apparently", because when I enlarge the canvas, I discovered that Photoshop is somehow retaining the precropped data and filling that in when I expand the canvas. This goes unnoticed when there is little significant detail in the filled in data. The solution is to crop the image (at the full size of the image as loaded) before resizing with the Canvas Size. The example below is of an image expanded using generative fill and then cropped (at an earlier time). I opened the image (2400x1513 pixels) and then created the red blocks at the 4 corners. I added a background layer, then increased the canvas size by 440 pixels in height and width with the anchor point at the center. I made the background blue for contrast. The resultant image shows where the "hidden" image was displayed. (Ideally, in this case, I would want the blue border to start at the outside of the red blocks.)
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