'Got data flow before stream-start event' in framecppchanneldemux

Using more recent versions of pygobject (3.28.3) and gstreamer (1.18.x), I'm getting gstreamer warnings when a framecppchanneldemux is used:

(python3:2214307): GStreamer-WARNING **: 08:06:28.292: ../gst/gstpad.c:4618:gst_pad_push_data:<framecppchanneldemux1:L1:FAKE> Got data flow before stream-start event