Created attachment 189634 [details]
Demonstration of the problem

SUMMARY
The brightness control slider for an external monitor (Philips 27M2N5500) connected via DisplayPort (USB-C) is available immediately after boot but consistently disappears from the system settings/tray after 2–5 minutes of uptime.

STEPS TO REPRODUCE
1. Boot into CachyOS.
2. Confirm the brightness slider for the Philips 27M2N5500 is visible in the UI.
3. Use the system normally for a few minutes.
4. Check the brightness control again.

OBSERVED RESULT
The slider for the external monitor disappears. The monitor is no longer recognized as a dimmable device in the desktop environment.

EXPECTED RESULT
The brightness slider should remain available and functional throughout the entire session.

SOFTWARE/OS VERSIONS
Operating System: CachyOS Linux 
KDE Plasma Version: 6.5.5
KDE Frameworks Version: 6.23.0
Qt Version: 6.10.2
Kernel Version: 6.19.0-2-cachyos (64-bit)
Graphics Platform: Wayland
Processors: 12 × 11th Gen Intel® Core™ i5-11400H @ 2.70GHz
Memory: 16 ГиБ of RAM (15.3 ГиБ usable)
Graphics Processor 1: NVIDIA GeForce RTX 3060 Laptop GPU
Graphics Processor 2: Intel® UHD Graphics
Manufacturer: HP
Product Name: Victus by HP Laptop

ADDITIONAL INFORMATION
This behavior is unique to CachyOS. On Fedora, the slider remains stable and functional.

Output of ddcutil detect after the slider disappears:
ddcutil detect
Invalid display
   I2C bus:  /dev/i2c-10
   DRM_connector:           card2-eDP-1
   EDID synopsis:
      Mfg id:               BOE - BOE
      Model:                
      Product code:         2733  (0x0aad)
      Serial number: [REDACTED]        
      Binary serial number: [REDACTED]
      Manufacture year: [REDACTED]
   This is a laptop display.  Laptop displays do not support DDC/CI.

Display 1
   I2C bus:  /dev/i2c-13
   DRM_connector:           card1-DP-2
   EDID synopsis:
      Mfg id:               PHL - Philips Consumer Electronics Company
      Model:                27M2N5500
      Product code:         49976  (0xc338)
      Serial number: [REDACTED]
      Binary serial number: [REDACTED]
      Manufacture year: [REDACTED]
   VCP version:         2.2