Save files as UTF-8 with BOM.
BOM forces Notepad++ to identify file as UTF-8.
Make sure that whatever software is consuming these files knows to skip the BOM. Most modern software knows to do it for many years but some systems that retain ancient legacy software pieces may not.
Without BOM Notepad++ needs to guess the encoding and sometimes it guesses wrong.
This forum has multiple discussions about this issue and the desire to be able to override this guessing with some default.
Since ANSI is a subset of UTF-8, in the absence of any special symbol within the file a UTF-8 is also an ANSI file and Notepad++ defaults to ANSI over UTF-8.