No idea besides filing a bug with an example file (btw, can you attach a small XCF that reproduces the problem, with instructions to create JPG and PNG from it).
Otherwise see if exporting to some 3rd format (TIFF, TGA, BMP) does the right colors and then convert with an external tools (IM or else).
Otherwise see if exporting to some 3rd format (TIFF, TGA, BMP) does the right colors and then convert with an external tools (IM or else).