A thing which makes a user stop and think "What? Why can't I do that? I can do that in any other thing." violates the Principle of Least Surprise, which, As I Understand It, is a fairly commonly held principle of (good) UI design.
From your question, it sounds like the Cut option is disabled only when the whole field is selected, which means that from a user's perspective, selecting one additional character can inexplicably disable functionality. With no explanation, and extrapolating from incomplete data, this could lead a user to conclude that "sometimes cut and pasting just breaks in this".
The general form of this principle is so general that it's hard to find specific recommendations to the contrary, but, this is from the OS X Human Interface guidelines, in the section on 'Consistency':
Is it consistent with people’s expectations? Is it consistent with people’s expectations? Does it meet the needs of the user without extraneous features? Does it conform to the user’s mental model?
Seemingly arbitrarily disabling functionality is not consistent with user expectations.
And, more specific to your needs:
When a user initiates an action, always provide an indication that your app has received the user’s input and is operating on it. Users want to know that a command is being carried out. If a command can’t be carried out, they want to know why it can’t and what can be done instead.
There's also a brief section in the GNOME3 Human Interface Guidlines entitled "Keep the User Informed", which includes the following:
The user should never have to guess about the status of the system or of your application.