Created attachment 24524 [details]
Solution to reproduce
I included a simplified solution with XML Deserialization unit tests that succeed on UWP but fail on Android and iOS.
It seems that if there are [XmlElementAttribute(Order = 0)] attributes on both base and derived classes that the order is interpreted different on Mono vs .NET. This is especially a problem since we have large generated files with types for Serialization.
So the order is different for serialization and deserialization