QJsonObject repetitive read/write of a key (large) results in error

Repetitive read/write of the same key to a QJsonObject results in the error:

QJson: Document too large to store in data structure

How to reproduce:

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

I call this procedure from the GUI:

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

Note that "a1" holds most of the content of object.
First ~ten calls are successful, the object is fine.

the next call results in the error:
QJson: Document too large to store in data structure

Write is not performed

Using "take" to remove the "a1" key from obj before writing it back results in the same behavior.