Bing base maps don't appear to be working any more in our Explorer maps. It works online when using AGO to create the map services and it works fine in Collector. However when we navigate to Explorer maps with Bing basemaps we get an error that it "can't find the the Bing key".
We had this similar problem over a year ago and an eventual update to the Explorer for iOS fixed the issue. Perhaps we are seeing the same problem rear its head again. iOS 11.3 came out recently and its about the same time this started happening.