How to enforce so Notepad++ uses UTF-8 every time I create a new file

Nothing is wrong. It is just, how Notepad++ is working. Files are actually saved as UTF-8 w/o BOM.

Whenever an ANSI-encoded text file is opened in Notepad++, editor displays its correct encoding (ANSI). However, after actually saving that file, it will be saved as UTF-8 w/o BOM, if Apply to opened ANSI files option is checked. To verify this, one must create a new text file in the Windows Explorer, open it in Notepad++, change contents, save and reopen. It will be recognized as encoded in UTF-8 w/o BOM.

Tip: There's a fairy easy way to force Windows Notepad to always save files in UTF-8:

• solution at answers.microsoft.com,
• solution at superuser.com.

It requires some tweaks in Windows registry, but allows you to have all files always encoded in UTF-8, no matter, what editor you're using.

To show the encoding options dialog, go to Save As... and check Edit filter settings.

In order to avoid the slowness of Save As..., you could use a macro like this:

Sub SaveAsUtf8
    dim aUrl()
    dim fileProps(1) as new com.sun.star.beans.PropertyValue
    fileProps(0).Name = "FilterName"
    fileProps(0).Name = "Text (encoded)"
    fileProps(1).Name = "FilterOptions"
    fileProps(1).Value ="UTF8,CRLF,Liberation Mono,en-US,"
    oDlg = createUnoService("com.sun.star.ui.dialogs.FilePicker")
    oDlg.setMultiSelectionMode(false)
    oDlg.initialize(array(1))
    oDlg.execute
    aUrl = oDlg.getFiles()
    If UBound(aUrl) > -1 Then
        thisComponent.storeAsURL(aURL(0), fileProps())
    End If
End Sub

Set it to a hotkey or toolbar button by going to Tools -> Customize.

It could be modified to use a global variable and save to the previously used location.

UTF-8 WITHOUT the BOM, which I believe is called ASCII/US

No, this produces ASCII-encoded text, which will destroy most Unicode characters.

I do not see any filter options that can save without a BOM from LibreOffice. Instead, there are various command line tools such as iconv that can remove the BOM.

If you have some time, the best solution may be to create a Python or Java macro to read the Writer document and write to a file without the BOM. It could be done in perhaps 30 lines of Python code, or twice that much Java code. Note: I would not recommend doing this in Basic because of its poor file handling functions.