null_sink.monitor channels error

I have a specific use case where I create a null sink with 2 channels (FL,FR) to be able to record one output on the FL channel and another output in the FR channel like so :

pactl load-module module-null-sink sink_name=record channels=2 channel_map=FL,FR
pactl load-module module-loopback source=FirstSource sink=record channel_map=FL,FL remix=no
pactl load-module module-loopback source=SecondSource sink=record channel_map=FR,FR remix=no

and then recording the record.monitor stream parec --format=s16le -d record.monitor | oggenc -b 192 -o "StereoRecording.ogg" --raw -

It used to work perfect with PA, with the only first source on the left channel and only the second source on the right channel. With PW both sources are on both channels. Can you have a look pls?