I attempted to recreate the unusual case you saw when formula level error management was turned on and value(textinput1.text)+value(textinput2.text)=0. However, my experiment produced a result of value(textinput1.text)+value(textinput2.text)=300. Is it possible that something else is causing the problem?
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@TitiwaiDo you mean if you turn off that feature, you will get the real sum of 300? Could you share your test steps and screenshots?
In my experience, if your form is in Edit mode and the Items property is empty, whatever you input in the data card, it always returns blank, so please check if your form mode and Items property. Further, you can add two text input boxes out of form to test and check if your formula work fine.