So I followed this procedure:
Created a hosted layer.
Created a webmap with the above layer.
Went to the Web Editor interface and use the above, used the above map. The table offers me to Calculate a field for selected records:

Then I proceed with the "Create App" function (while in the Web Editor interface). This took me to the instant app builder where I compiled an App with the editable functions. All stock settings as the instant app allowed.
However, the field Calculator is not available in the app:

Am I missing a setting somewhere or is the function not supported?