I have discovered that if any text is tabbed, and then you modify it in any way (bold, italicize, increase font) it will no longer be tabbed when synced to Evernote, but will still appear tabbed in NixNote. The HTML tags are getting messed up when modifying tabbed text.
Steps to recreate bug (this is just a very specific example, but similar steps should produce similar results) :
1) Create a new note in NixNote
2) Write "test" on the first line
3) Insert tab
4) At this point if you sync, everything is fine
5) Modify "test" in any way (bold, italicize, etc.), then resync
6) "test" will no longer be tabbed on Evernote's side but still tabbed in NixNote