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Date & Time: N/A
Frequency: Only seen it happen once
World name: Midgardsormr
Character name: Luna Puella
Retainer name: N/A
NPC name: Khloe Aliapoh
Monster name: N/A
Class/Level: Level 65 Machinist
Party or solo: Solo
In-game time: N/A
Area and coordinates: Idyllshire x5.7 Y6.1
Housing: N/A
Steps: N/A
1. Complete Wonderous Tails and visit Khloe
2. Accept "Timeworn Kumbhiraskin Map" as a reward
3. Finalize decision, you will receive the other rewards like usual (EXP) but no pre-warning for having a map in your inventory already, and so you receive no map and by extension - no other rewards.

I know it's expected that you won't get the map but there is no "are you sure?" dialogue box or alert to prevent you from accepting the reward regardless, so it's possible to not get any reward at all.