In OutGauge "Display1" is 16 char string which shows car's fuel in percent and in absolute value (litres). After car change it only gives percentage and misses the litres value and is no longer 16 chars long. I kept driving and the litres value eventually appeared again when it was 10.0L. Can't be anything else than bug I guess, am I right?

In fact that is Laps, not Litres.

That part of the display appears after you have done a lap, so LFS knows how much fuel you use in a lap and can estimate the number of laps left in your tank.