When I set the font’s grid spacing to other values, the arrow key stepping in the width and sidebearings only try to increment by 1 or +10, and the input is rejected especially under coarse grid (imagine making pixel font in 100 units grid).
True, I noticed that too, but I was suffering quietly through it until now. Will check.
I have noticed that this applies to kerning too!
 It seems it already does.