Just checked on my Live Singleplayer by spawning in all necessary ingredients and learning the recipes, and you might be on to something. I only have one DLC, so it’s easy to see in the screencap that the Exceptional version is not available. This is after removing/replacing the thrall both before and after restart. The recipe doesn’t show up for me. On the off chance it matters, the blacksmith I spawned in was from the Relic Hunter faction.
Didn’t think to check that, but it’s a smart call. Spawned one in from the Heirs of the North faction and the recipe does display.
*Also spawned a Nordheimer blacksmith from the Exile faction, but the recipe does not display. It has to be a “true” nordheimer.