Class not included in build even though its in link.xml

We are using the Amazon unity SDK ( https://github.com/aws/aws-sdk-net ) and are having the following error during initialization on android (Mono2x scripting backend, not il2cpp):

 07-05 10:10:05.166 16954 16982 I Unity   : MissingMethodException: Method not found: 'Default constructor not found...ctor() of Amazon.Util.Internal.PlatformServices.NetworkReachability'.

However, this class is part of AWSSDK.Core.dll, and we have the following entry in our link.xml:

 <assembly fullname="AWSSDK.Core" preserve="all"/>

What are we doing wrong? Is there any log that shows what types were put in the build?