Use the drop-down menu to the left of the track > Split Stereo Track. Leave the two tracks Left and Right. It’s a simple matter to Make Stereo Track when you’re done.
Koz
That was a bug in some old versions of Audacity. In certain circumstances, dragging a stereo track would only drag one channel and not the other, but that was unpredictable and unintended (a bug). For stereo tracks, dragging is supposed to move both channels.