Ran into a glitch a few months ago regarding one of the mysteries in Glowecestrescire, at first, after giving the letter to the correct person, the quest giver simply disappeared when returning to him to complete the quest.
Went back to it today (10/11/23) hoping it would have been fixed after a few months, to find that now only one of the NPC's shows up to offer the letter to and after trying to give it to him to see what happens, the quest giver on the wall still disappears anyway.
As a result, I'm unable to complete the mystery whilst playing on PS4 and have found no workarounds as of yet.
Leaving the area and returning offers up the same result each time and I have seen others having the same problem with no fixes yet.