For best results, use the following guidelines for creating text file sitemaps:
- You must fully specify all URLs in your sitemap as Google attempts to crawl them exactly as you list them.
- Your text file must use UTF-8 encoding.
- Your text file should contain nothing but the list of URLs.
Personally, if we need a sitemap created outside WordPress (does it automatically), then we use XML Sitemaps (https://www.xml-sitemaps.com/)