I feel like I found a bug but I'm not sure - when I do then Shift +G I don't think the behavior is correct.
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?