So I abandoned the Raid Plan quest in an attempt to try and complete it post-maintenance.
Upon further experimenting with it, I found that if you try to accept the quest without the processed armor the chatbox pops up with the dialogue identical to what the centaur leader says when you talk to him back on the hill. I'm thinking an NPC ID got switched somewhere along the lines and is producing an error.
If so, it definitely shouldn't have taken four and a half weeks to have found this...
Hoping someone picks this up and fixes this ASAP because there are a TON of players unable to proceed with the game because of this.