SUMMARY

Normally I would find two sliders (one for each monitor) in the brightness applet to change monitor brightness. 
When the display gets powered off (either after just locking or full sleep), the slider for one of the display (always the same) is missing, while the other works normally. Plugging, unplugging, powering off the displays, etc. does nothing, but logging out and in from plasma fixes the issue. 

The issue doesn't occur every time, but does occur.

The issue doesn't seem to occur if the display is connected over hdmi instead of displayport, but I can confirm it doesn't always happen.

It is not a `ddc` issue as the `ddcutil` command still works

STEPS TO REPRODUCE
1. Have an "LG HDR WFHD" monitor connected via displayport
2. Suspend and resume
3. Check if the slider is still there
4. Retry until its not

OBSERVED RESULT

The slider is gone

EXPECTED RESULT

The slider should still be there

SOFTWARE/OS VERSIONS
Operating System: Fedora Linux 42
KDE Plasma Version: 6.3.5
KDE Frameworks Version: 6.14.0
Qt Version: 6.9.0
Kernel Version: 6.14.9-300.fc42.x86_64 (64-bit)
Graphics Platform: Wayland
Processors: 16 × AMD Ryzen 7 7700X 8-Core Processor
Memory: 31.0 GiB of RAM
Graphics Processor: AMD Radeon RX Vega


ADDITIONAL INFORMATION

The output of `ddcutil detect` is 

> Invalid display
>    I2C bus:  /dev/i2c-3
>    DRM connector:           card1-HDMI-A-1
>    EDID synopsis:
>       Mfg id:               MST - MS Telematica
>       Model:                MStar Demo
>       Product code:         48  (0x0030)
>       Serial number:        
>       Binary serial number: 1 (0x00000001)
>       Manufacture year:     2012,  Week: 10
>    DDC communication failed. (getvcp of feature x10 returned Error_Info[EIO in ddc_write_read_with_retry, causes: EIO])
> 
> Display 1
>    I2C bus:  /dev/i2c-6
>    DRM connector:           card1-DP-3
>    EDID synopsis:
>       Mfg id:               GSM - Goldstar Company Ltd (LG)
>       Model:                LG HDR WFHD
>       Product code:         23456  (0x5ba0)
>       Serial number:        
>       Binary serial number: 496642 (0x00079402)
>       Manufacture year:     2021,  Week: 2
>    VCP version:         2.1

The problematic monitor is the second one (LG), but using the command `ddcutil --model="LG HDR WFHD" setvcp 10 66` I can see that it's still working with ddc, just the slider is missing.