Why quarter step increment on brightness, volume and backlight is not working on a Touch Bar MacBook Pro?
Tap the arrow on the left of the system area to expand the full list of system buttons, then use the brightness increase and decrease buttons from there.
(If you've removed them, you might need to go to View → Customize Touch Bar and add them back in.)
Since those buttons have the exact same function as non-Touch Bar models, you can use the alt + shift combination to get the quarter-step changes.