If Photoshop is not resampling, and you change the resolution, the image size has to change. This is not a bug.
If you are not resampling, the number of pixels remain the same. When you change the resolution, Photoshop must either make the pixels small if the resolution is increased, or make them larger if the resolution is decreased. When pixels get smaller so that there are more of them per inch, the physical size of the image must get smaller. When pixels get larger, the physical size of the image must get larger.
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