NPC doesn't recognise an achievement and cannot purchase an item as a result

[PickledthePirate]

I'm not given the option to purchase the Fleeting Rays orchestrion roll from Chachamun at the highbridge in Eastern Thanalan, despite meeting the requirements to unlock her expanded inventory. The rest of the expanded inventory is missing too.

Date & Time: 1st Feb 2021 8.10pm
Frequency: Constant
World name: Moogle
Character name: Tulip Markell
NPC name: Chachamun
Party or solo: Solo
Area and coordinates: Eastern Thanalan (x22.2, y21.2)
Housing:
Steps:
1. Own the "Curtain Call" achievement for completing \\\\'Attack on Highbridge: Act III\\\\'
2. Speak to Chachamun to purchase from their expanded shop
3. Their inventory remains default and nothing new can be purchased

[MrThinker]

I'm not given the option to purchase the Fleeting Rays orchestrion roll from Chachamun at the highbridge in Eastern Thanalan, despite meeting the requirements to unlock her expanded inventory. The rest of the expanded inventory is missing too.

Date & Time: 1st Feb 2021 8.10pm
Frequency: Constant
World name: Moogle
Character name: Tulip Markell
NPC name: Chachamun
Party or solo: Solo
Area and coordinates: Eastern Thanalan (x22.2, y21.2)
Housing:
Steps:
1. Own the "Curtain Call" achievement for completing \\\\'Attack on Highbridge: Act III\\\\'
2. Speak to Chachamun to purchase from their expanded shop
3. Their inventory remains default and nothing new can be purchased

IIRC, its not actually owning the achievement that gives you the expanded inventory, but having completed the FATE successfully, where it remains like that for everyone before the FATE chain restarts. Once the chain restarts, you lose the inventory.

[Canadane]

IIRC, its not actually owning the achievement that gives you the expanded inventory, but having completed the FATE successfully, where it remains like that for everyone before the FATE chain restarts. Once the chain restarts, you lose the inventory.

This is correct.
It's part of the FATE events, unrelated to the act of completing it itself.