QResource file engine returns read-only memory for some resources even with MapPrivateOption

Here is a crappy sample how it happens:

if (QFile f(id); f.open(QIODevice::ReadOnly) &&
  (dat = f.map({}, f.size(), QFileDevice::MapPrivateOption)))
{
  *dat = {};
}

Will SEGV after a second resource (e.g.":/resource.png") is loaded in, but not the first. I used this method for lazy reading instead of loading a resouce into a QByteArray. I don't see how I could be responsible.