Summary
modify_contents item modifier can create overstacked items inside a container, like a shulker box. (The overstacked item can then be spawned by burning the dropped shulker box.) Also note that overstacked items no longer crash the game.

Steps to Reproduce

1. Get a shulker box with one item

   /give @s minecraft:shulker_box[minecraft:container=[{item:{id:stone},slot:0}]]
   

2. Run the following command

   /item modify entity @s hotbar.0 {function:modify_contents,modifier:{function:set_count,count:99},component:container}
   

Expected Result
Shulker box does not have one stack of 99 stones.

Actual Result
Shulker box has one stack of 99 stones.