When saving a TGraphAsymmErrors with negative value to svg it produces a box in svg with negative width. In the svg definition it specifies boxes with negative width should not be drawn so these do not show up. When saved as pdf the expected output is seen.
Fix would be to shift the x value of the box in the svg file and have a positive width to be compliant with modern svg renders. Note the box is incorrectly drawn in ImageMagick so the fix should not be tested viewing with this program, instead a modern browser can be used as these correctly render the svg.
Example code to produce the issue attached, running with root test.C saves the svg and pdf files to see it working correctly in the pdf and failing in the svg.