I would suggest that a bug is a deviation from advertised or reasonably expected* functionality. Microsoft advertised (in the Windows 7 help file) Notepad as:
Notepad is a basic text-editing program and it's most commonly used to view or edit text files.
No advertisement is made of functionality that maintains only one open copy of any file, and my ‘reasonable expectation’ of a ‘a basic text-editing program’ would not include such a feature either. As such, I do not believe that what you describe can be reasonably called a ‘bug’, but a ‘feature request’.
This is not a bug but by design.
Notepad doesn’t have the concept of locking files (as in “proper” word processors like Word etc.). Therefore what you are seeing is to be expected.
Notepad is only really meant for quick edits to simple text files (such as .bat file). If you want this sort of functionality then you need to install and use something like Notepad++.
It sounds like you’re relying on doing important work with notepad. This isn’t wise. Notepad does not perform a file lock on its active files which means any other process is free to open and read/write to your file even while you’re working with it in notepad.
You should find an alternative text editor for your work if that is an issue for you but the reality is it’s easier to check yourself if the file is open (if it’s not shared and potentially network users have access to it) than it is to try and enforce file locking.