Sorry, Photographer question concerning Lightroom

It is too small to really see what is happening. But, my best guess is, if you zoom in to the color photo there is an odd purple-y line in that place and the b/w conversion is rendering it as black.
It has to do with how light waves enter a camera lens when there is a dark/bright contrast such as in her shirt. The effect is called chromatic aberration.  It gets rather technical so I'll let you google it.
If after googling that doesn't seem right try posting a close up of the area in a larger size so it is more visible.

after you convert to b/w, go to the colors and pull up the magenta to darken the shirt a bit and lower the purple levels that will probably fix it. It probably is chromatic aberration. Actually, I think her shirt needs to be darker in the b/w so the values are the same. Be careful doing this though as sometimes those colors are in the faces and you can make the faces look very strange doing this.