Problems Calculating Area in Field Maps

I've set up a smart form in ESRI Field Maps for one of the field crews at my job to fill out. It's up to them to fill out most of the fields but there are two which should be populated through calculated expressions, the year and area (acres). My year calculation works perfectly and is written as follows:

return Year(Now())

Now, the expression for the area calculation I have written using smart forms is valid and returns a value when I test it in arcgis online. However, when I use Field Maps to create a new feature, the area field does not populate while the year field does. Here's the expression for area:

return area($feature, 'acre')

Does anyone have any ideas of what might be going wrong?