I'm seeing the exact same behavior using desktop application for Mac OS X 10.13.6, TurboTax Premier 2019 (Application: 2019.r25.040, Build Date: Apr 10, 2020 at 1:53:52 PM). Per Arizona Ind Formset 2019.11495).
In the Forms view, AZ Form 140, Line C5 is calculated. The only User Entered field is C1.
In my case, I need to enter a non-zero amount in line C5 because I am taking the tax credit for one of my gifts. (Edited to remove work-around that didn't work.)
