Hello,
Try restarting your PC a couple of times and see whether you can sign in using your PIN.
Alternatively, this issue could be caused by the keyboard connected to your device. Check whether any keys on the keyboard are stuck, especially the Shift keys (left or right Shift) or the Alt keys. And,
If you are using an external keyboard, try unplugging it and see whether the same issue occurs. With the keyboard disconnected, use your mouse to open the On-Screen Keyboard and then enter your PIN.
This will help me determine whether the physical keyboard is causing the issue.
Additionally, there have been a few reports of LG monitor settings causing similar issues. The joystick on the back of the monitor can be used to access the monitor's on-screen settings menu and then disable this setting.