Using the following block of code, definitionList does not contain any data. The object is created but with a collection of null properties.
var manafest = client.Api.Destiny2_GetDestinyManifest().Result;
var definitionListJson = new WebClient().DownloadString($"https://www.bungie.net/{manafest.JsonWorldContentPaths["en"]}");
var definitionList = JsonConvert.DeserializeObject<AggregateDestinyDefinitions>(definitionListJson);
Your usage of JsonConvert implies usage of Newtonsoft.Json. BungieSharper uses System.Text.Json and does not use the naming attributes that Newtonsoft looks for.
Using the following block of code, definitionList does not contain any data. The object is created but with a collection of null properties.