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The value reads http://dservices.arcgis.com/arcgis/services/V6ZHFr6zdgNZuVG0/JapanPrefectures2018/WFSServer, which is not correct. The value needs to be http://dservices.arcgis.com/V6ZHFr6zdgNZuVG0/arcgis/services/JapanPrefectures2018/WFSServer.
I attempted publishing a WFS Layer using a hosted feature layer and it produces the same error. I also tried publishing a WFS service from ArcGIS Pro, which works perfectly, I'm guessing because the Namespace is set as an option when publishing via Pro.
Any ideas? Is this a bug? Something I've missed? Something else?
I tested this on an in-house WFS, created from a hosted feature layer in ArcGIS Online, and was able to reproduce the issue when sending a GetFeature request. I've logged this as BUG-000129387. I would recommend getting in contact with Esri Support to attach your customer number. Let me know and I can assist with this process.
Hope that helps,