I know this is an old question, but for anyone else looking for the answer like I was today, I just converted my IDictionary<string, object> to ExpandoObject with the below extension method and it work as expected.
public static dynamic ToExpando(this IDictionary<string, object?>? dict)
IDictionary<string, object?> expando = new ExpandoObject();
if (dict == null)
foreach (var (key, value) in dict)
expando[key] = value;