Hi
Had the issue where the migration from Evernote to OneNote using the OneNote Importer lost some titles, seeming random.
I looked at the XML in a text editor, and noticed that some XML tags on those with no title was different (missing some XML, which set the encoding, which I think is the issue).
So, I did the following:
1) export from Evernote the notes you want to migrate (manually).
2) open the resulting .enex file in textedit
3) do a manual find/replace in textedit, replacing the seemingly "faulty" tag <![CDATA[<!DOCTYPE en-note SYSTEM "http://xml.evernote.com/pub/enml2.dtd"> with <![CDATA[<?xml version="1.0" encoding="UTF-8" standalone="no"?><!DOCTYPE en-note SYSTEM "http://xml.evernote.com/pub/enml2.dtd"><en-note>.
4) Save the file in textedit
4) Open the OneNote import tool.
5) Re-run the import with the OneNote import tool, selecting the newly saved file (with the find/replace carried out).
Evernote team, this appears to be your (export) issue, not Microsoft's import issue.