This has happened quite a few times now, I have a bamboo farm and a sugarcane farm with rails running underneath and hopper carts circling underneath the blocks where the items are planted.  I have looked through all the hoppers and chests and they do not show up there, there have not been any creepers that have blown the area up, they just vanish.  I am not sure if it has something to do with the circle pattern of the rails but in another part of the world I have a similar system to catch mop drops under a zombie spawner and the carts do not disappear, in this instance the carts bounce back and forth on straight rails.

Clients connecting to the world are:

Xbox One X

Windows 10 1803

Android 9