there are no QBI limitations like you describe in any of the desktop or other versions. any entry on line 20 code Z is only so Turbotax later asks about the 3 components income. wages, UBIA. In forms mode, these entries go on the separate line 20Z (right below line 20) through use of the quickzoom link which you should see in forms mode
in step mode for line 20 code Z just enter the income or loss
then there will be another screen asking 3 questions about 199A (QBI)
then continue on the next page will allow entries for the components (after checking the box there's a drop down to enter the amount
by the way the 751 income (20AB) is also QBI and should be enter on the other income line under the business income line
