When items which can stack inside a single block are picked up and placed back down, they reduce to 1, or they return to the state they are in when they are first placed, like a cake with 1 piece left will be restored to 6 pieces after picked up and placed back down-
These items include:

minecraft:sea_pickle
minecraft:pink_petals
minecraft:snow
minecraft:turtle_egg
minecraft:candle
and minecraft:cake

This also applies to items in some mods like the HandCrafted mod
e.g. handcrafted:white_plate