HttpClient reads incomplete or malformed string - json

Question

Im trying to figure out which part of my app (Xamarin Forms and proxy written in PHP) is buggy. Firstly I thought that my proxy (written in PHP) is working incorrectly with long set of data (ie. json containing 1.300.000 characters) and returns malformed response, but every single request with Postman gives me correct JSON, which is successfully decoded with third-party tools. So I assume, proxy is working well.
The problem is (I guess) with decoding response in my Xamarin Forms (2.0.0-beta.22) app. I'm using HttpClient to read response with this code:
response.EnsureSuccessStatusCode();
var entries = new List<HistoryEntry>();
var content = await response.Content.ReadAsStringAsync();
_loggerService.Error(content);
response is just GetAsync response from HttpClient. The problem is: content is randomly incomplete/malformed. Saying this I mean last character is missing (}) or JSON keys/values have additional " character, which breaks everything. Unfortunately, I can make exactly the same requests many times and once it works, once not. I found out that this behavior happens only with large set of data (as I mentioned before, long JSON string).
Is there any possibility that ReadAsStringAsync does not wait for full response or in any way alters my response string? How can I find the reason of wrongly downloaded data?
EDIT 21.05.2019:
Just copied valid JSON (available here: https://github.com/jabools/xamarin/blob/master/json.txt) and returned it from Lumen app by response()->json(json_decode(..., true)) and still the same result. Hope someone will be able to reproduce this and help me with this issue :( More informations in comments.
Used this code in C#:
Device.BeginInvokeOnMainThread(async () =>
{
for (int i = 0; i < 10; i++)
{
var response = await client.GetAsync("<URL_TO_PHP>");
//var response = await client.GetAsync("https://jsonplaceholder.typicode.com/photos");
var content = await response.Content.ReadAsAsync<object>();
Debug.WriteLine("Deserialized: " + i);
}
});