Web maps containing OS Maps API Web Map Tile Service (WMTS) basemaps falsely treat the layer as secured in ArcGIS Field Maps (iOS) if the service has an access key and was originally added as a content item before being added to the web map.

Workaround

1. Instead of initially adding the Web Map Tile Service (WMTS) as a content item, add it directly into Map Viewer when creating a new map using 'Add layer from URL'.
2. Add the URL in the same way as before, appending the custom parameter key.

iOS ArcGIS Field Maps opens the web map without error.

Steps to Reproduce