When a square image is used and the *-progression-dimension.maximum are equal, then the image fails to scale and it overflows the box limits.

For example this would reproduce the problem:

      <fo:block>
        <fo:external-graphic src="url(file:///D:/dev/fop/src/fo/300x300.JPG)"
            inline-progression-dimension.maximum="50mm"
            content-width="scale-down-to-fit"
            block-progression-dimension.maximum="50mm"
            content-height="scale-down-to-fit"
            border="solid 5pt red"/>
      </fo:block>


Workaround:
Different values for *-p-d.maximum give expected result.
Even minor changes to the dimension would resolve the issue.
For example in the previous code if one of the progression-dimension.maximum was set to 50.01mm the image would scale correctly.