Start a new spreadsheet, add some text and save it as XLS/XLSX.
=> 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 18.104.22.168 & 22.214.171.124 / Windows 7. In 126.96.36.199 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.