I had the knowledge and I deleted it with the Elixir of Oblivion and now I can't get it back even with the rented pistol from Merio. Can anyone else confirm this? Same goes for Jellyfish.
I got just the same problem. Deleted the knowledge ib Calpheon Library and now can't get it back IDK is it a bug?
P.S. I have found a solution for this: use a Practice Matchlock and CROUCH. Got both knowledges to S very fast!
To upgrade this knowledge to S did not have to delete it with the new knowledge system, but a hunting Matchlock didn't work only the Practice Matchlock that I rented from Merio