Drawing edges of merged mesh - Merging 2 cylinder results in black mesh

I don’t know if its a bug or if I just use it the wrong way.

I have 2 cylinder where I draw the top (EdgeRenderer) and the side (renderOutline) edges.
First when they intersect each other its breaks the edge rendering a bit.

I tried merging them as a solution, but then they get completely black.
My playground:

Don’t add the merged mesh to the scene, it is automatically added: