That's rather a question you should bring to the GStreamer folks. For the Qt side you can then use QMediaPlayer as shown here with a custom pipeline.
Hi and welcome to devnet,
Glad you found a solution and thanks for sharing !
One thing you can do is to take a look at the release logs to see what has changed.
However from your description you may have found a regression.
Did you already check the bug report system ?
@SGaist please an example to guide me on the right way,because i don't know how to do it