Time: Always
Frequency: Always
World name: Bahamut
Character name: Drar Satirof
NPC name: Retainer
Monster name: -
Class/Level: Any
Party or solo: Any
In-game time: Always
Area and coordinates: Idyllshire - haven't tried other zones
Steps:
1. Deposit collectible fish into retainer (in my case, Illuminati Perch)
2. In retainer inventory; right click and choose option 'Retrieve from retainer'
3. Fish disappears from retainer's inventory, then reappears a second later
Note: Other items (non collectible's) still transfer fine.