I've heard that feedback before, but I couldn't quite understand what was wrong. I'd really appreciate your help in troubleshooting.
My understanding is that Shift + G should be absolute positioning, so 12G will bring you to the twelfth line.
I do deviate slightly from default VIM in that relative lines are one, so the line count starts at 0, respective to your cursor. Is there a chance that this plays into your experience of the bug?