SUMMARY
When waking up a device from sleep, it will show the time and wallpaper. You can use the mouse or the keyboard to "focus" and begin logging in.

This by itself is fine, but in order to unlock the device with a fingerprint, this action has to be taken first. This means you cannot simply open a laptop lid and use your fingerprint to unlock.

In my case (and from what I have observed in general, for others too) the fingerprint that my device uses is on the opposite hand to the one I use the trackpad with.

This case would also allow for opening, say, a laptop lid and pressing the power button, but keeping your finger on the power button and having it unlock the device (perhaps after a (configurable?) delay). This is how it works on macOS for example.

STEPS TO REPRODUCE
1. Setup fingerprint on a device
2. Put device to sleep (i.e. on a laptop, close the lid)
3. Wake device
4. Try to unlock device with fingerprint.
5. Nothing will happen.
6. The lock screen has to be "focused" (unsure of the correct word) to show the username and password field, and only then can the fingerprint scanner be used (and works without issue).

OBSERVED RESULT
The fingerprint reader cannot be used until the lock screen is "focused" and the login user and password field are displayed.

EXPECTED RESULT
The fingerprint reader can be used as soon as the screen comes on and the screen locker is displayed.

There is a message under the passwordtelling you that you can use your fignerprint, but it should be possiible to use your fignerprint earlier than this as well. It works this way on macOS for example (unsure of other operating systems).

SOFTWARE/OS VERSIONS 
Linux/KDE Plasma: 6.9.5 Linux Zen
KDE Plasma Version: 6.1.0
KDE Frameworks Version: 6.3.0
Qt Version: 6.7.1

ADDITIONAL INFORMATION
- I am aware that you cannot use the fingerprint on first boot, this issue is for after a screen is locked / system it put to sleep, not on bootup.
- Unsure if this is a regression, but it has been the behaviour since at least Plasma 6.0.5, as that is the first time I used a computer other than a Mac with a fingerprint reader.