This is a bug, I guess:
After creating a new key command, the “lengthen duration by grid value” gives me a doubling of the note value. After restarting Dorico it works as normal again.
I guess it is the same for shorten duration by grid value.
I haven’t tried if other changes in “deeper” parts of Dorico is creating the same bug too.
I can’t reproduce this. Can you give me a bit more detail on what exactly you’re doing? What key command are you creating a shortcut for, and what is the shortcut?