I didn't realize you were saving in ACR. I tried that and got a screwed up jpeg as well when converted to Grayscale, Q12, jpeg. I did notice something though - down at the bottom where the blue hyperlink is, click it, and change the colorspace to sRGB and try saving as Q12 jpeg. See what happens. It worked ok on mine. Originally it was saving in a Grayscale Gamma mode.
*****should have went back and reread the Adobe forum before posting, *****