一度String型で取得してから、デシリアライズ（JSON→コレクション）する。

using System.Text.Json;

[FunctionName("XXX")]
public static async Task<HttpResponseMessage> Run(
    [HttpTrigger(AuthorizationLevel.Anonymous, "get", "post", Route = null)] HttpRequestMessage req,
    ILogger log)
{
    dynamic dynamicdata = await req.Content.ReadAsAsync<object>();
    
    // NG
    List<XXEntity> xxEntities = JsonSerializer.Deserialize<List<XXEntity>>(dataR?.addEntities);
    
    // OK
    string s = dataR?.xxEntities.ToString();
    List<XXEntity> addEntities = JsonSerializer.Deserialize<List<XXEntity>>(s);

}