Entering characters by using ALT with decimal number differs on virtual console and konsole (GUI)

I have an issue with ALT + ASCII value representation in OpenSUSE Tumbleweed with xfce. When in virtual console, I can press ALT key + ASCII value of a character and get the desired output.

Example ALT + 65 equals ‘A’.

However the same method does not work in GUI.

Linux obviously knows what is the ASCII representation of a character, let’s say ‘A’ which is 65. When I type this command in virtual console: ‘showkey -a’ and then press the key ‘A’ it shows this:

A 65 0101 0x41

This problem was originally discovered using a USB scanner, however can be replicated with just a keyboard.
Previous thread is here: https://forums.opensuse.org/t/usb-scanner-ds4208-emulate-keypad-option

I have searched a bit on the web. They way entering Unicode characters (and thus ASCII characters) by having the ALT-key pressed down and then typing the numerical representation seems to be Microsoft Windows based. Many Linux user thus ask how to do the same on their system. The answer being most cases to point to the way this can be done in Linux. Thus assuming that the user can adapt his/her habit to a new one: not using Alt, but something different (like Shift-Ctrl-U, or the compose key). But that will not help in this case, because the scanner can not be talked into using another way to send it’s output.

It seems that there is no real Linux way to get the interpretation of pressing Alt while typing the decimal value of a character functioning.
But there is still the question, why is it functioning on the virtual console? And of course: can we copy that behaviour to the desktop (Xfce for the OP, but KDE does the same).)