Sidebearings & keyboard nudge increments, strange behaviour

I have my grid system set up like so:

And I want to use the up / down arrow keys to adjust sidebearings:

If I press the up or down keys, the counter doesn’t change immediately to the next increment (+8 units). Instead, it appears as though nothing is happening, and the key needs to be repeatedly pressed to see any change (40, 40, 40, 40, 40, 40, 40, 48)

It would be nice if the value incremented straight to the next step (40, 48, etc)

CleanShot 2022-01-21 at 17.19.10

What happens when you do a “Shift+Arrow”?

Shift+Arrow behaves as expected:
CleanShot 2022-01-24 at 09.19.11