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 Water Strider.

I also tried melee for about 15 minutes and it wouldn't pop, first musket shot on the Annoying Jellyfish quest (https://bdocodex.com/th/quest/3530/1/) from Merio using the Practice Matchlock and I got it. Pretty sure you HAVE to use the matchlock.