Multiple 4K videos in a GStreamer / QML app = black flickering - qt

Question

I'm developping a QML application allowing to display up to four 4K videos, with a Decklink 8K Pro and the qmlglsink element.
The hardware setup is the following:
4 4K cameras
1 DeckLink 8K Pro
1 Dell Precision with 2 Intel Xeon E5-2698 (40c/80t), 256GB of memory and an Nvidia GeForce GTX 1080
When displaying the 4 videos at the same time in my app, I frequently have black frames (I see a black flash and then the video is back). When displaying 3 videos, blinks are much less frequent, and I don't think I've seen them when showing only 1 or 2 videos.
A few interesting things:
if I start 4 instances of Gstreamer with one 4K video in each, there's no blink
if I start 4 instances of my app with one 4K video in each, there's no blink
say I'm showing 2 times camera A and 2 time camera B (I also have some hardware to duplicate video streams). When a blink occurs, it does for the two video views of that camera (A or B).
It looks to me like a threading problem, but adding a queue element to my pipeline does not fix the problem. Setting QSG_RENDER_LOOP to threaded with qputenv doesn't either.
Is there anything else I can do?
Here's some code used to create the pipeline:
// Create the source element (see below for the makeElement method):
GstElement* decklink_src = VideoView::makeElement("decklinkvideosrc", { {"device-number", 0}, {"mode", "2160p2997"}, {"profile", 4} });
// Create the pipeline element (see below for the generatePipeline method):
GstElement* pipeline = generatePipeline(_source, m_videoViewItem->findChild<QQuickItem*> ("videoView"));
// Schedule rendering:
m_window->scheduleRenderJob(new SetPlaying (m_pipeline), QQuickWindow::BeforeSynchronizingStage);
GstElement* VideoView::makeElement(const std::string& _elementName, const std::map<std::string, QVariant>& _params)
{
GstElement* element = gst_element_factory_make(_elementName.c_str(), nullptr);
for(const auto& attribute : _params)
{
const char* name = attribute.first.c_str();
switch (attribute.second.type())
{
case QVariant::Type::Int:
g_object_set (element, name, attribute.second.toInt(), nullptr);
break;
case QVariant::Type::String:
gst_util_set_object_arg (G_OBJECT(element), name, attribute.second.toString().toStdString().c_str());
break;
default: break;
}
}
return element;
}
GstElement* VideoView::generatePipeline(GstElement* _src, QQuickItem* _parent)
{
GstElement* pipeline = gst_pipeline_new(nullptr);
GstElement* queue = gst_element_factory_make ("queue", nullptr);
GstElement* glupload = gst_element_factory_make ("glupload", nullptr);
GstElement* glcolorconvert = gst_element_factory_make ("glcolorconvert", nullptr);
GstElement* sink = gst_element_factory_make ("qmlglsink", nullptr);
g_assert (_src && queue && glupload && glcolorconvert && sink);
gst_bin_add_many(GST_BIN (pipeline), _src, queue, glupload, glcolorconvert, sink, nullptr);
gst_element_link_many( _src, queue, glupload, glcolorconvert, sink, nullptr);
g_object_set(sink, "widget", _parent, nullptr);
return pipeline;
}