Inside https://invent.kde.org/multimedia/kdenlive/-/blob/master/data/effects/copychannelstostereo.xml: 0;1;2;3;4;5;6;7;8;9;10;11;12;13;1;15 should read 0;1;2;3;4;5;6;7;8;9;10;11;12;13;14;15 Fix needs to be applied in multiple places. Gettext files should be updated accordingly.
Confirmed, "13;1;15" should be "13;14;15", but it looks like there's a bigger problem: swapchannels.xml, copychannelstostereo.xml, and channelcopy.xml all seem to point to the same actual effect in MLT (channelcopy) with varying parameters. Someone probably needs to take a look at all three of them together.