They put a bandaid on it to allow us to harvest shadow nodes, but it's still really broken for anyone who didn't scribe the recipe during the questline. I put in the bug report and made a post under the bug forum thread a week or so ago, but nada... keep doing it I guess.
It is supposed to raise to 25 though. If he's stuck at 20, then it is supposed to auto increase to 25 when he harvests a node. If that isn't happening for some, then they need to fix it or allow GMs to help with that.
It might not be any comfort to you, but after the fix my shadow harvesting went straight to 25 after I harvested a node. It only happened after harvesting a normal node though, did not happen from harvesting a shadow node.
don't feel bad, i just got the xpac, got to 120 adventure, then did the entire crafting line. i have the staff. i get spammed at the end of every normal harvesting node saying its setting my skill to 25 prospecting. i still dont have the shadow prospecting skill. im guessing its linked to my internet going out a few times this week while i was doing the crafting line.
You can buy the book which gives shadow prospecting from the Sanctus Seru TS merchant in the TS instance if you didn't get it/destroyed it. Price is 12s iirc.