Seconds scalar is always null in vue-apollo

Question

I'm trying to pass an int value as seconds scalar to my graphql-backend. The frontend uses vue-apollo and the backend is written in dotnet core using graphql-dotnet.

When I call the mutation via playground, everything works fine. Neither does my vue app throw any error/exception nor works the mutation. The mutation is called in the backend, but the value for s is null (and so 00:00:00 as it is a TimeSpan). The backend is written in dotnet core 3.1 but as I said the convertion from a given number to TimeSpanSecondsGraphType works via playground.

frontend code

My mutation looks like

updateS(
  sId: ID = null
  s: s = null
): SType

My input type s looks like

type s {
  s: Seconds = null
}

And my SType looks like

type SType {
  s: Seconds
}

Working example call (via playground)

mutation {
  updateS(s: "08d924e8-69fb-425c-8130-de7342a2ca2a", s: {
    s: 3600
  }) {
    s
  }
}

This is the payload from chrome when I call the mutation in my vue-app (not working):

{"operationName":"updateS","variables":{"sId":"08d924e8-69fb-425c-8130-de7342a2ca2a","s":{"s":3600}},"query":"mutation updateS($sId: ID!, $s: s!) {\n  updateS(sId: $sId, s: $s) {\n   s\n__typename\n  }\n}\n"}

Edit - Added backend code

dotnet core mutation and models

This is my mutation in the backend:

Field<SType>(
                "updateS",
                arguments: new QueryArguments(
                    new QueryArgument<IdGraphType> { Name = "sId" },
                    new QueryArgument<SInputType> { Name = "s" }
                ),
                resolve: context =>
                {
                    var sId = context.GetArgument<Guid>("sId");
                    var s = context.GetArgument<S>("s"); // s.S is always {00:00:00} when called from vue app. 
                    return repo.UpdateS(sId, s);
                });

My SInputType:

public class SInputType : InputObjectGraphType
    {
        public SInputType()
        {
            Name = "s";
            Field<TimeSpanSecondsGraphType>("s");
        }
    }

And my S.cs:

public class S
    {
        public Guid Id { get; set; }
        public TimeSpan S { get; set; }
    }

Any suggestion?