@Ragnaguard I had not at the time I generated this thread, as I had yet to hit an RNGesus jackpot and land it. However, in the interest of of inquiry and for the sake of clarity and betterment for the community, I spawned in a few Potions of Natural Learning this afternoon.
I can now confirm that at least some, but not all, learned Feats found on the Isle of Siptah will be lost/unlearned with both the Admin Panel respec, AND also the Potion of Natural Learning!
The results were very sporadic, and seemingly even random. There was a loss of some, but not all learned Siptah feats for each method. For example:
With the Admin respec I lost → devolved creature statues insignias, statues, trophies (etc).
With Potion of Natural Learning I lost → eldarium chest recipe, eldarium braziers recipe, abyssmal remnant recipes.
Feats which remained following the consumption of either or both → stygian palisades, stygian tent, Forgotten clan banner, island hunter, iron leg hold trap, linguist: Nemedian, daggers of the grey ones.
Note-I am sure that there are more recipes, but the hour is late here, and I cannot recall what else there was. I will further update this thread/post as needed.
EDIT-while I cannot recall the name of the feat, I have also lost my ability to craft ‘red’ weapons, learned by defeating the Acid Wight boss near the Tower(?). Again I cannot recall their names.