The final libreOffice write document must be converted to PDF for publication submission. THe numerical anchor endnotes are converted to roman numeral anchor endnotes upon this conversion. I used an .odt document and a .doc document with the same results.
As you don’t give information on your LibO version, OS and what you did in detail it is not possible to simulate your observation and give you a precise information.
If the problem occured with the pdf-export function, you can - as a possible workaround - try to get a free / free for evalution pdf-writer and create your pdf-file this way. Check the internet.
Perhaps also check the settings under Tools > Footnotes/Endnotes.