Keyboard shortcuts use the Num keys for other functions.
In some cases or many, you cannot reprogram them to set note duration (or anything else) through the UI.
There is a workaround floating around for reprogramming Num keys by using the UI to export the current definitions; using an editor to change the definitions; and using the UI to import the changed definitions. It works; I've done it but I don't know where the instructions are.
Comments
This is a known issue.
and a a duplicate too, and fixed in 4.0.1