I had the quest activated by chance (never bothered with AF body), so I checked the NPCs.
The quest works, both Vemalpeau and Najjar gave me the correct dialogue and the latter mentioned the Mique's paintbrush key item.
The thing is, they immediately revert back to their default lines, so if you check them twice you won't get anything about the Under Oath quest; you might've already gotten the AF quest dialogue and forgot about it.
FFXI: Searain WAR75 PLD75 NIN75, Garuda server, 2003~2008
Nasomi: Searain PLD75 DRK75 NIN75 COR75, 2014~