While there is no player within 16x16 blocks of an item entity, the entity starts to linearly gain negative y motion while retaining its position. Even if the block beneath the item entity is destroyed for example by using "setblock", the item continues to retain its original position. This behavior stops as soon as a player moves within 16 blocks of an item entity.

The negative y motion can also be verified using the data command:

/data get entity @e[type=item,sort=nearest,limit=1] Motion[1] 

This behavior was recorded in the "item_entity_bug.mp4" video.

Also, every fourth item inside a chunk seems to consistently demonstrate this behavior regardless of the distance to the nearest player. This behavior was recorded in the "item_entity_bug_2.mp4" video.