Start a new spreadsheet, add some text and save it as XLS/XLSX. Reopen spreadsheet. => Font has changed to Arial. Font is preserved if you choose a font like Linux Biolinum, however Carlito is also substituted as Calibri. Observed using LO 5.4.3.2 & 4.2.0.4 / Windows 7. In 4.1.0.4 the default font is Arial.
This is a feature-not-bug - we do this to ensure the widest possible metric compatibility and round-trip fidelity. If we save Liberation as Liberation - then it will simply not be present on typical Windows machines - and as such the user will get mangled metrics, and a broken display. You can check out the substitutions in VCL.xcu in the source tree =) we have lots of them, if some are missing - we should expand that.
*** Bug 139473 has been marked as a duplicate of this bug. ***