Unsolved QJsonObject repetitive write - possible bug

[AndreyMor]

I have a large QJsonObject (size of ~40 Mb when saved to json file).

I call this procedure from the GUI:

{
  QJsonObject jRC = obj->js["a1"].toObject();
  obj->js["a1"] = jRC;
}

Note that "a1" holds the most of the content of that object.
After about ten successful calls I get an error:
QJson: Document too large to store in data structure

This is just a demonstrator, I have the same problem in my code.
Any way around this issue?

I am using Qt 5.8.0 (Windows 7, MSVC2013 )