I used de-synchro, but doesn't re-summon monsters from graveyard.
What did I miss here?
This happened a few times, not just with Moonflower, despite that cards are clearly in the graveyard.
I've been playing without de-synchro for that matter, but imo it's better to play that then adding more cardians.



Turn 1 (Player 1):
Set The White Stone of Ancients
Turn 2 (Player 2):
Activate Flower Gathering
> Spawn Flower Cardian Willow, Zebra Grass, Paulownia, Pine (Face up ATK)
> Shuffle deck
Zebra Grass activate effect (negated)
Activate Allure of Darkness
> Draw Flower Stacking, Gathering
> Banish Pine
Activate Flower Stacking
Special summon Clover with Boar by tributing Pine
> Draw Willow with Calligrapher
> Destroy The White Stone of Ancients
Special summon Willow with Calligrapher
> Draw Willow
> Special summon Willow
Willow activate effect
> Return Willow from graveyard
> Shuffle deck
> Draw Paulownia
Synchro summon Moonflower by tributing Willow, Calligrapher, Paulwnia
> Activate effect
> Draw Maple with Deer
> Special summon Maple with Deer
Maple with Deer activate effect
> Draw Flower Stacking
> Discard Flower Stacking
Synchro summon Moonflower by tributing Moonflower, Zebra Grass, Clover with Boar
> Activate effect
> Draw Flower Gathering
Activate Super Koi Koi
> Special summon Cherry Blossom with Curtains
> Banish Fairy Tail - Snow, Soul Charge

De-synchro starts here:
Synchro summon Moonflower by tributing Moonflower, Cherry Blossom with Curtains, Maple with Deer
> Activate effect
> Draw Allure of Darkness
Activate Allure of Darkness
> Draw De-synchro, Cherry Blossom with Curtains
> Banish Paulownia
Activate De-synchro on Moonflower
> Returns Moonflower to extra deck
> Doesn't spawn three monsters from graveyard

As far as I can tell, if you special summon a "Flower Cardian" with an effect that says "ignore summoning conditions" it is not considered as being properly special summoned, which means you cannot revive it from your graveyard.

See your step here:

Synchro summon Moonflower by tributing Willow, Calligrapher, Paulwnia
> Activate effect
> Draw Maple with Deer
> Special summon Maple with Deer

Maple with Deer says: "Must first be Special Summoned (from your hand) by Tributing 1 "Flower Cardian" monster, except "Flower Cardian Maple with Deer"."

Since you special summoned it with Moonflower, it wasn't special summoned properly, so you cannot revive it. That's why you don't get your materials back with De-Synchro.