Issue with doubling tracks to pan the lead vocal left and right in chorus

Hi Everyone - I’m having an issue when doubling the lead vocal in the chorus of a song and panning those tracks left and right. So there is a lead vocal that is centered, which I copy to a new track and pan left, then copy and paste the lead again to a third track and pan right. The strange thing is that half way through the chorus the sound changes, suddenly getting “wider” like the original centered lead vocal drops out of the mix. If I listen to each track individually they sound fine. Listening to the Lead and the left-panned track together is fine. But when I add the third right-panned track to the mix the issue arises. I’ve gone over every setting and they are all the same, so I am really stuck as to what the issue is with the third track. Anyone experience this type of issue and have advice?