This is caused by the HW decoder and fakesink. The HW decoder has only 4 buffers in the bufferpool, when the four buffers are sent to fakesink as a whole batch, the fakesink will hold them as the last frame. So th decoder will stop and wait for fakesink to release the buffers.
We need to let fakesink to release the buffers. Please try the following pipeline:
gst-launch-1.0 filesrc num-buffers=20 location=/opt/nvidia/deepstream/deepstream-5.0/samples/streams/sample_1080p_h264.mp4 ! decodebin ! queue ! mux.sink_0 nvstreammux name=mux batch-size=4 width=1920 height=1080 batched-push-timeout=40000 ! fakesink enable-last-sample=0