Double chests may swap items between sides when reloaded, and they randomly assign items from hoppers to one side or the other

Some large chests (ONLY large chests) will randomize the items inside sometimes when leaving and rejoining the world. The large chest will open but won't show the animation when you're opening and closing it sometimes with any large chest (not all). 

 

(Game modes: Survival, Creative, and Adventure)

This is happening in the versions of 1.21, 1.21.20.21–(Preview), and maybe Future updates.