If FLS does not provide data for some reason, the tracker may send zeroes instead. Those zeroes will be interpreted by the program as valuable data. That is why non-existent fuel fillings and thefts may appear.

To avoid this situation, set in FLS properties X lower bound like '0.0001'. So, values lower than this will be considered as invalid and ignored in calculations. Note that X lower bound should not be higher than minimal X value from the calculation table.

As an alternative, you can use validation to solve this problem – validation type 'Not-null check'. In this case you will have to create two sensors based on the parameter of FLS.