Hi Neil,
Unfortunately, based on the current version of Runtime iOS SDK 10.2.5, the label will not show up in webmap. And this issue is reproducible in Quartz.
We have already logged an enhancement request for the Quartz Runtime: [ENH-000092338 AGSMap doesn't display label of the web map in ArcGIS Online] in our Early Adopt Community: https://earlyadopter.esri.com/project/feedback/view.html?cap=59D261350FB84BE48C01CF04BF2A3AA1&uf=E4E...