The aircraft bizarrely started to climb at the FAF
Sure it wasn't just going for the missed approach altitude, that being the next 'hard' one in the box if you don't put anything in after the FAF?
(Just curious, it happened to me during my intro. to FMCs in a B737-300 in 1986, training sector, training capt. let it all continue as a demonstation of what can happen, sent back to read the books, emphasis on 'hard' altitudes etc.! - dumped VNav and used Fl Change with speed intervention thereafter.)