SQ318 indeed landed back to Changi at 1900hr+
Sources told was that there was navigation degeneration few hours into the flight that departed at 1246pm
Fuel dump was to reduce the weight and the risk of overshooting the runway (usually by destination, fuel would have been used and thus landing at destination will not have weight concern). Fuel dump could also reduce the severity of combustion in event of unfortunate accident during landing.