Mono vocal track still playing out of one headphone after panning down

Hello,

I am trying to mute one section of a vocal track, but it is still playing audio after I pan it down. In Waveform, I converted the samply type from stereo to mono with 50% / 50% for R & L mixing. In the screenshot below you can see the levels on the R headset are high when playing over the panned down section. 

Any guidance would be greatly appreciated!

Thanks,

Charlotte