The bug

When using /fill to fill multiple blocks with blocks that have block entities, the command only creates a block entity for the first block. All other blocks will reference the same block entity at the wrong position.
This can lead to invisible blocks, if the rendering of that block is bound to the block entity (e.g. chests or shulker boxes).
This also makes it impossible to access the block entity's inventory, if it has one.
Reloading the world, the inventory can be accessed, but the block entity is only temporarily created after trying to access it. Only writing to the block entity (e.g, putting an item into the chest) will then finally make the game store the block entity permanently.

How to reproduce

1. Run /fill ~ ~ ~ ~2 ~ ~ chest
   → only one chest will be visible, the other one references the block entity in the wrong location and can't be opened
   → /data get block will reveal that the chest references the other chest's block entity