I noticed the same on my flight today. And then I figured that I didn't enter the step climbs in the FPLN, which once done gave me a correct fuel figure without being in the negative.
This of course makes sense as the Simbrief OFP is calculated taking into account the step climbs and the winds at the different cruise altitudes, so if you programm your MCDU with the first cruise FL only, it will think that you will fly the whole flight at FL320 for example, and thus have a bigger fuel burn.