Installed 5.1.7 Hera on my laptop (Lenovo Yoga 6 13ARE05); everything except brightness control seems to be working great. Brightness keyboard controls and System Settings -> Power -> Display Brightness slider are syncing/working, but do not affect the screen brightness. Any help greatly appreciated.
If you still can't get the backlight to work, I recommend sending a bug report to Ubuntu's kernel team as outlined in Ubuntu's Backlight Debugging Wiki.
You can also try color correction with xrandr, but this is more of a hack that simulates brightness adjustment without actually controlling the backlight. This won't save power or accurately preserve colors.