I'm pretty sure it's cut content, not available in the game. At least I think so, could also just be a bug preventing them to trigger.

When I did romance Karlach which I assume would work since she's not Minthara, I also never got any of these romance banters and I definitely would remember this one;

  • Karlach: Hey, Shaddy, got a quick question about the best way to treat an infernal burn.
  • Shadowheart: Shaddy? Really?
  • Karlach: Hearty? Shadsy? Or maybe just The Fringe?
  • Shadowheart: 'Shadowheart' will do just fine, thanks. Love is making you fanciful.
  • Karlach: You might be right.