Brightness adjustment Fn keys stopped working in 18.04

I realize there are potential duplicates of my question. However, I have scoured different pages and nothing solved my problem. I have recently done a fresh install of Ubuntu 18.04 (had the previous long-term release) and simply cannot get the brightness keys to trigger brightness changes.

Details:

• Ubuntu x64 18.04.
• Not using Wayland.
• No integrated graphics in the processor.
• There is no screen brightness slider when I click on the battery icon. I had a brightness controller showing on the previous Ubuntu installation on this exact same hardware - why was it hidden in this iteration of Ubuntu?
• NVIDIA proprietary drivers in use as per the following image:

CPU:

*-cpu
          description: CPU
          product: Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz
          bus info: cpu@0
          version: Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz
          capabilities: x86-64 fpu fpu_exception wp vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf tsc_known_freq pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb invpcid_single pti ibrs ibpb stibp tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm mpx rdseed adx smap clflushopt intel_pt xsaveopt xsavec xgetbv1 xsaves dtherm ida arat pln pts hwp hwp_notify hwp_act_window hwp_epp cpufreq

GPU:

sudo lshw -C display
  *-display                 
       description: VGA compatible controller
       product: GP106M [GeForce GTX 1060 Mobile]
       vendor: NVIDIA Corporation
       physical id: 0
       bus info: pci@0000:01:00.0
       version: a1
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress vga_controller bus_master cap_list rom
       configuration: driver=nvidia latency=0
       resources: irq:122 memory:dd000000-ddffffff memory:90000000-9fffffff memory:a0000000-a1ffffff ioport:e000(size=128) memory:c0000-dffff

This is my current grub configuration:

GRUB_DEFAULT=0
GRUB_TIMEOUT=10
GRUB_TIMEOUT_STYLE=menu
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"

With these settings on (I did restart grub and the system after setting them), I can listen for the Fn+Brightness combinations:

acpi_listen 
video/brightnessdown BRTDN 00000087 00000000
video/brightnessup BRTUP 00000086 00000000

With this, I at least know that pressing the brightness change keys (via Fn plus F8 for lower brightness or F9 for higher) is recognized by the system. If I'm not mistaken, these events are supposed to trigger certain brightness change scripts.

However, it seems nothing is called, even though those events are assigned.

Brightness down: video/brightnessdown BRTDN 00000087 00000000

cat /etc/acpi/events/asus-keyboard-backlight-down 
# /etc/acpi/events/asus-keyboard-backlight-down
# This is called when the user presses the key brightness 
# down button and calls /etc/acpi/asus-keyboard-backlight.sh for
# further processing.

event=video/brightnessdown BRTDN 00000087 00000000
action=/etc/acpi/asus-keyboard-backlight.sh down

Brightness up: video/brightnessup BRTUP 00000086 00000000

cat /etc/acpi/events/asus-keyboard-backlight-up
# /etc/acpi/events/asus-keyboard-backlight-up
# This is called when the user presses the key brightness 
# up button and calls /etc/acpi/asus-keyboard-backlight.sh for
# further processing.

event=video/brightnessup BRTUP 00000086 00000000
action=/etc/acpi/asus-keyboard-backlight.sh up

Both events define as action the following file:

vim /etc/acpi/asus-keyboard-backlight.sh

This is its content:

KEYS_DIR=/sys/class/leds/phy0-led\:\:kbd_backlight


test -d $KEYS_DIR || exit 0

MIN=0
MAX=$(cat $KEYS_DIR/max_brightness)
VAL=$(cat $KEYS_DIR/brightness)

if [ "$1" = down ]; then
        VAL=$((VAL-1))
else
        VAL=$((VAL+1))
fi

if [ "$VAL" -lt $MIN ]; then
        VAL=$MIN
elif [ "$VAL" -gt $MAX ]; then
        VAL=$MAX
fi

echo $VAL > $KEYS_DIR/brightness

Furthermore, if I inspect the directory in variable $KEYS_DIR, there are the following contents:

sudo ls /sys/class/leds/phy0-led
brightness  device  max_brightness  power  subsystem  trigger  uevent

The brightness and max_brightness files are empty.

Everything appears to be correctly setup, but I can't understand why doesn't the brightness change, why doesn't it remain saved after reboot if I use an alternative program such as brightness-controller or xbacklight and why is the brightness slider missing?

Thank you very much.