Auto merge edge slide deleting vertices

Hi, I'm new to blender and I'm following a tutorial, but when I'm using auto-merge and alt/opt selecting a vertices loop then double tapping G to edge slide the vertices on top of each other, the vertices are being deleted rather than merging.

Any help would be appreciated. I basically just want to clean up the geometry to create a low poly look. I've used the skin modifier to create the geometry. I've used both edge and vertices snapping.