SUMMARY
I just updated KDE Neon to Plasma 5.26, and I noticed that I fail to unlock the screen.
Fortunately, there is the option to show the entered password in clear text, and a virtual keyboard.
Therefore, I noticed that the password text box does not accept any input from navigation keys: Backspace and arrow keys do not have any effect.
Same for Enter, and therefore I could not unlock.

I managed to unlock by entering my password as usual, and then pressing the Enter button of the virtual keyboard.

Of course, my buttons are physically fine.
After unlocking, I can type as usual.

STEPS TO REPRODUCE
1. Lock the screen. (Meta + L)
2. Press buttons on the physical keyboard.
3. Press buttons on the virtual keyboard (after clicking “virtual keyboard” in the lower left corner of the lock screen).

OBSERVED RESULT
Only letters from the physical keyboard are accepted.

EXPECTED RESULT
All buttons are accepted.

SOFTWARE/OS VERSIONS
Operating System: KDE neon 5.26
KDE Plasma Version: 5.26.0
KDE Frameworks Version: 5.98.0
Qt Version: 5.15.6
Kernel Version: 5.15.0-50-generic (64-bit)
Graphics Platform: X11

ADDITIONAL INFORMATION
I use neo2 keyboard layout.
This layout offers a second set of navigation keys on the main field.
However, both the second set and the “traditional” navigation keys are affected the same way.
The second set of number keys works fine.
A typical symptom of applications without neo2 support is that the second set of navigation keys and number keys are broken, and type symbols like “~/}_()-:”, or are simply dead.
More ridiculous applications present gibberish like “KP_EnterKP_LeftKP_RightKP_BackspaceKP_4KP_5KP_6KP_Comma”...
I conclude that the problem with the lock screen is not that neo2 is broken.