This behaviour is specific to Ubuntu only (32-bit, 12.04/13.10, with LibreOffice 4.0.2.2), I haven’t encountered it on my Windows machine.
It amounts to this: all formatting (that is all possible formatting) in .odt documents is gone after the file is closed and reopened. Upon reopening the file presents itself in pure, default, DejaVu Sans Mono 10 pt with absolutely no trace of previous formatting. It is also recursive: let’s say I redo everything, after saving, closing and reopening, everything is back to bare-bone defaults.
This makes .odt in Ubuntu useless for me. Frustratingly, .doc/.docx and .rtf work just fine. What I found is that “exporting” from .doc/.docx to .odt, as opposed to creating and building on an original .odt, makes the problem disappear - you get a fully functional document that preserves everything you throw at it.
Test samples:
link:odt original
link:pdf snapshot
link:docx copy