Guys, look how Saturn got affected by the Mother Flame
He instantly went back to his human form, the "uggh" makes it look like he feels bothered by its sole presence in a water tank
PS: Oda never made Sanji use his flames or go serious against Saturn
Meanwhile Sanji was managed to do this with Diable Jambe: He forced Nasjuro to change his form in order to stop the flames
Also look how Sanji here is seeming to be more effective than even Luffy in Gear 5 has been against any of the Gorosei
Luffy's attacks against Saturn basically meant nothing, Warcury is blocking everything, but Sanji when he goes a bit serious he sets one of them in flames forcing them to transform right after the attack?
So the Gorosei either have a weakness to overall flames or Sanji's flames indeed has something to do with the Mother Flame to some extent we still don't know yet
This are starting to be explained perhaps?