Under normal circumstances, if an item is a block level below you and you fill in the space it's in with solid blocks, it will shuffle to a nearby open space.  Then, if no space is available (that pit is filled), the item will bounce upward and be collectable by the player.

I noticed repeatedly just now (mining ores under bedrock where it can't be reached unless the player forced it out with blocks) that the item instead disappeared rather than responding to the filled space.

I tested this further and made a 2-block thick floating "cup" and filled in the middle. The block glitched downward (falling, resetting back upward, falling) and eventually fell out the bottom instead. 

I then made a thicker platform (4 blocks thick) and tried again. The block took awhile but again dropped out the bottom and a bit diagonally from its original position.

The usual behavior in these cases is the item would rise to the top. In my case a bunch of items are instead being tossed down into the void.