Interesting, Darko.
Have I read somewhere that you've fitted a replacement fuel tank level sender? If so, do you know the part number of the sender or fuel pump assembly that you have fitted?
I ask because there are two types of level sender available, one for the 34L tank and one for the 42L tank. The principle of operation of both level devices is that the resistance of the sender changes dependant on the fuel level. The difference being for the 34L and 42L senders are that for one (I don't recall which) the resistance decreases when fuel is added, and for the other, the resistance increases when fuel is added. In your case, if you had the incorrect sender fitted, then as fuel was drained the fuel gauge could move the other way, as the resistance is reporting to the instrument cluster that more fuel is being added. Correct sender = full tank, full gauge. Incorrect sender = full tank, empty gauge.
I'm not sure if it's even possible to fit a 34L sender or fuel pump to your 42L tank, but it may be worth investigating.
Regards,
Matt.