Code Stages currently only support older language levels of .NET languages. Especially C# is currently only supported in v. 5.0, which was released in 2012. Modern constructs like initializing auto-implemented Properties were introduced in later versions but are features expected by programmers to be available per default.
As BP is using the native .NET framework to compile Code Stages on runtime into dynamic assemblies, the latest version supported is C# v. 5.0. More recent versions can easily be supported by using Microsoft's Roslyn compiler platform, which is provided as a NuGet package:
https://www.nuget.org/packages/Microsoft.CodeDom.Providers.DotNetCompilerPlatform/It should even be possible to configure the language level dynamically so old code can be compiled in the same way as it currently is.