When using Buffer, a popup will not display a value in a Web AppBuilder App. However, it is shown in an AGOL map. Centroid(), which was released with Arcade 1.3, works properly.
WAB Developer Edition 2.6 (JSAPI 3.22)
Arcade: 1.3 (JSAPI ??)
This will return the xmin value of the extent of a buffer.
var geom = Geometry($feature)
var buf = Buffer(geom, 1, 'miles')
var ext = Extent(buf)
The resulting popup in AGOL is correct. It does not appear in WAB.