Blizzard has confirmed that the 18-slot version of Pattern: Bottomless Bag should not be dropping in Season of Discovery. Later today, the 20-slot version of Pattern: Bottomless Bag will be added to the <Lost and Found> vendor, Rix Xizzix in Booty Bay. Players who have received the 18-slot version of Pattern: Bottomless Bag will be able to trade this in for the 20-slot version.
In addition to the Pattern: Bottomless Bag, players will be able to exchange the 18-slot version of the Bottomless Bag for the 20-slot version of the Bottomless Bag at Rix Xizzix.