SUMMARY
I'm not sure if this is an xdg-desktop-portal bug, a KWin bug, a Plasma bug, or even an OBS bug.

I have two monitors:

* primary monitor: left ultrawide on HDMI
* other monitor: right full HD on Display Port (with an HDMI->DP adapter)

When I open OBS Studio, click on the + Add Source button, select Screen Capture (Pipewire), and then choose New Virtual Output, the virtual output that gets created shows up on the Display Configuration KCM to the right of the other screens, but my panel moves to the right monitor.

Additionally, the radio button used to choose a primary monitor in the Display Configuration KCM disappears.

In the kscreen-doctor outputs list, it doesn't seem that the second monitor is considered the primary monitor, though, as the priorities are correctly are set as 1 -> 2 -> 3.

STEPS TO REPRODUCE
1. Have two monitors
2. Set left monitor as primary one
3. Open OBS Studio
4. Click on the + Add Source button
5. Select Screen Capture (Pipewire) in the screen selection dialog
6. Choose New Virtual Output
7. Inspect System Settings -> Display & Monitor -> Display Configuration

OBSERVED RESULT
1. Panel moves to other monitor
2. Radio button to choose Primary monitor disappears

EXPECTED RESULT
1. Panel stays in the same place
2. Radio button to choose Primary monitor doesn't disappear (not sure if intended ???)

SOFTWARE/OS VERSIONS
Operating System: Fedora Linux 42
KDE Plasma Version: 6.4.5
KDE Frameworks Version: 6.19.0
Qt Version: 6.9.2
Kernel Version: 6.16.11-200.fc42.x86_64 (64-bit)
Graphics Platform: Wayland
Processors: 16 × AMD Ryzen 7 5700G with Radeon Graphics
Memory: 32 GiB of RAM (31.2 GiB usable)
Graphics Processor: AMD Radeon RX 7600
Manufacturer: Gigabyte Technology Co., Ltd.
Product Name: A520M K V2

ADDITIONAL INFORMATION
The first time I added a virtual output it also rearranged my screens, but I couldn't reproduce it afterwards.

KSCREEN-DOCTOR LOGS

~  → kscreen-doctor --outputs
Output: 1 DP-1 1fd1d8f7-5af8-4c88-8a2d-23a92c3e6121
        enabled
        connected
        priority 1
        DisplayPort
        replication source:0
        Modes:  1:1920x1080@60!  2:1920x1080@75*  3:1920x1080@60  4:1920x1080@60  5:1920x1080@50  6:1680x1050@60  7:1400x1050@60  8:1600x900@60  9:1280x1024@75  10:1280x1024@60  11:1440x900@60  12:1280x800@60  13:1152x864@75  14:1280x720@60  15:1280x720@60  16:1280x720@60  17:1280x720@50  18:1024x768@75  19:1024x768@60  20:800x600@75  21:800x600@60  22:720x576@50  23:720x480@60  24:720x480@60  25:720x480@60  26:720x480@60  27:640x480@75  28:640x480@60  29:640x480@60  30:640x480@60  31:720x400@70  32:1280x800@75  33:1600x900@75  34:1368x768@60  35:1368x768@75  36:1280x720@75 
        Geometry: 2048,0 1536x864
        Scale: 1.25
        Rotation: 1
        Overscan: 0
        Vrr: incapable
        RgbRange: Automatic
        HDR: incapable
        Wide Color Gamut: incapable
        ICC profile: none
        Color profile source: sRGB
        Color power preference: prefer efficiency and performance
        Brightness control: supported, set to 10% and dimming to 100%
        DDC/CI: allowed
        Color resolution: automatic (10), range: [8; 16] bits per color
        Allow EDR: unsupported
Output: 2 HDMI-A-1 67ac49b7-e735-4df5-bba1-db211eddccc2
        enabled
        connected
        priority 2
        HDMI
        replication source:0
        Modes:  37:2560x1080@60!  38:3840x2160@30  39:3840x2160@30  40:3840x2160@25  41:3840x2160@24  42:3840x2160@24  43:2560x1440@60  44:2560x1080@75*  45:2560x1080@50  46:1920x1080@75  47:1920x1080@60  48:1920x1080@60  49:1920x1080@60  50:1920x1080@50  51:1680x1050@60  52:1600x900@60  53:1280x1024@75  54:1280x1024@60  55:1440x900@60  56:1280x800@60  57:1152x864@60  58:1280x720@60  59:1280x720@60  60:1280x720@60  61:1280x720@50  62:1024x768@75  63:1024x768@60  64:832x624@75  65:800x600@75  66:800x600@60  67:720x576@50  68:720x480@60  69:720x480@60  70:640x480@75  71:640x480@60  72:640x480@60  73:640x480@60  74:1280x800@75  75:1600x900@75  76:1368x768@60  77:1368x768@75  78:1280x720@75 
        Geometry: 0,0 2048x864
        Scale: 1.25
        Rotation: 1
        Overscan: 0
        Vrr: Automatic
        RgbRange: Automatic
        HDR: disabled
        Wide Color Gamut: disabled
        ICC profile: none
        Color profile source: sRGB
        Color power preference: prefer efficiency and performance
        Brightness control: supported, set to 10% and dimming to 100%
        DDC/CI: allowed
        Color resolution: automatic (10), range: [8; 16] bits per color
        Allow EDR: unsupported
Output: 3 Virtual-virtual-xdp-kde-com.obsproject.Studio 42080557-1fd9-4175-bd3c-8f61448c2135
        enabled
        connected
        priority 3
        Unknown
        replication source:0
        Modes:  79:1920x1080@60*! 
        Geometry: 3584,0 1920x1080
        Scale: 1
        Rotation: 1
        Overscan: 0
        Vrr: incapable
        RgbRange: unknown
        HDR: incapable
        Wide Color Gamut: incapable
        ICC profile: incapable
        Color profile source: incapable
        Brightness control: unsupported
        Color resolution: unknown
        Allow EDR: unsupported