The beacon block, if you left a diamond, gold, iron, or emerald in a stack or not, when you reload the chunks, via reloading world, or going in and out of the nether, the beacon will eat the items inside. Entire stack.

Possibly due to no tile-entity data for the slot, causing the bug. Will not work if you just close and open the beacon, as the item will be still there.
Maybe fixed by either make the player drop the item, if they close it, via the X, or the 'Open Inventory Button', and make it not able to be hopper in, or add a entity data for the slot.

Can be annoying to someone who somehow left a diamond in the beacon, and came back to realize that the diamond had been eaten, even though they were checking around to see if they can use gold, or something else.

If you need video of it, I'll be happy to do so. Images are not enough just to describe it, as you may think I am faking it.