[Reg B2Qt 6.7.3 -> 6.8.3] glupload not supported in GStreamer pipeline

Inserting glupload as a preprocess element in Qt's GStreamer pipeline by setting

QT_GSTREAMER_OVERRIDE_VIDEO_CONVERSION_ELEMENT=glupload ! glcolorconvert

used to work in 6.7.3 but doesn't work in Qt 6.8.3. The pipeline is fully linked and playing, but the glupload element seems to create its own glcontext instead of using the one provided by Qt.