When constructing a KPI, if you use an account group, which is an Expense type group, that also contains an Income account, the calculation will start with the value of the Income account, and then deduct all the Expense account values and give a net result.

If the expenses are higher than the income, this will be a Net Loss, which is a negative amount (Net Loss), hence the KPI is reporting as a negative.

To correct this, build the Net Profit calculation actually in the KPI, so you will need to include the Account Groups by type. Then in the KPI Builder you would run as

(Expense + CoGS – Income)

To give a positive figure, followed by the rest of your calculation. This will correct the KPI result to being a positive value.