I have the same inconsistent behaviour. I have a PC which has this issue, but my laptop doesn't have it. 
They're both running the latest KDE Linux version.

Unfortunately on my main(In reply to Darryl from comment #0)
> SUMMARY
> When I use Apply Plasma Settings, Numlock preference is not applied.
> 
> STEPS TO REPRODUCE
> 1. Ensure Numlock On is turned on in System Settings
> 2. Go to System Settings - Login Screen
> 3. Apply Plasma Settings, enter password
> 
> OBSERVED RESULT
> Numlock is off.
> 
> EXPECTED RESULT
> Numlock On, 
> 
> SOFTWARE/OS VERSIONS
> Linux/KDE Plasma: KDE Neon Testing
> KDE Plasma Version: 6.5.90 (6.6 Beta) 
> KDE Frameworks Version: 6.23.0
> Qt Version: 6.10.1
> 
> ADDITIONAL INFORMATION
> I first brought up this bug on
> https://discuss.kde.org/t/plasma-login-manger-numlock-preference-not-applied/
> 43616/1
> 
> Here is a copy of my post:
> 
> I am not sure if this bug is with Plasma Login Manager, KDE Neon Testing, or
> the Qt version.
> 
> Plasma Login Manager, when I use Apply Plasma Settings, Numlock preference
> is not applied using KDE NEON Testing, Plasma 6.6 beta., however on my other
> testing of Plasma 6.6 Beta on CachyOS, the Numlock setting is successfully
> applied.
> 
> Both distros are on the same hardware:
> AMD FX-8370
> Nvidia GeForce GTX 750 Ti
> 
> Both using Wayland
> Both using Nvidia Driver: 580.126.09
> 
> KDE Neon Testing:
> KDE Plasma Version: 6.5.90
> KDE Frameworks Version: 6.23.0 (yes I know that hasn’t be released but
> that’s what indicated on About this System)
> Qt Version: 6.10.1
> Kernel Version: 6.14.0-37-generic
> 
> CachyOS:
> KDE Plasma Version: 6.5.90
> KDE Frameworks Version: 6.22.0
> Qt Version: 6.11
> Kernel Version: 6.18.5-1cachyos

Just updated my non-testing CachyOS to Plasma 6.6, installed Plasma login manger, tried to apply settings (tried a couple of times), and numlock failed to be applied.

Operating System: CachyOS Linux 
KDE Plasma Version: 6.6.0
KDE Frameworks Version: 6.23.0
Qt Version: 6.10.2
Kernel Version: 6.19.2-2-cachyos (64-bit)
Graphics Platform: Wayland
Graphics Processor: NVIDIA GeForce RTX 4070 Ti SUPER

Hi, also impacted on Arch just after switching from sddm to plasma-login.
I had a file /etc/sddm.conf.d/numlock.conf with these lines :
> [General]
> Numlock=on

and Numlock was correctly set to ON on startup.

But if I rename the /etc/sddm.conf.d/ folder to /etc/plasmalogin.conf.d/ (following Arch documentation), the config is not applied anymore. I need to keep it on /etc/sddm.conf.d
I'm using Arch plasma-login-manager 6.6.0-1

Cursor theme and size, is also not applied.

it seems to work here (arch linux with kde plasma 6.6.1):

/etc/plasmalogin.conf

[Greeter][Wallpaper][org.kde.image][General]
Image=file:///var/lib/plasmalogin/wallpapers/KDE Plasma Nebula 187.png
NumLock=on

btw: this is working after having applied plasma settings in system settings => login screen

I've managed to solve the issue for me on both KDE Neon Testing and CachyOS.
I disabled and uninstalled plasmalogin manager, and then searched for any leftover folder/files and deleted them /usr/lib/plasmalogin, /etc/plasmalogin.conf, /home/user/.local/plasmalogin, 
I then reinstalled, enabled, and applied plasma settings. Now Numlock and cursor theme is applied. 

I assume the leftover files in /usr/lib/plasmalogin from the previous version somehow interfered.

*** Bug 516226 has been marked as a duplicate of this bug. ***