If you had a loss in 2018 then there would be a negative QBI deduction that carries to 2019 and gets netted against any positive QBI deduction you get this year ... that is correct.
Here is the reasoning ... you got the income deduction last year against other income instead of the QBI so you must carryforward the unused QBI to the following year. If you also had a loss this year then the negative QBI carryforward would be increased. This is the flip side of the coin ... you don't get to have your cake and eat it too ... there is always red tape.
