Enemies' thrown weapons disappear when dropped

[Grond]

The small rocks thrown by goblins disappear if you pick up and then drop them. They do not disappear when you throw them.

EDIT: Weapons thrown by enemies also disappear if you pick up and then drop them. In both cases (rocks and weapons) the "you drop a X" message is not displayed.

[MssngrDeath]

Interestingly, small rocks gained by other means (like mining) stick around when dropped. Only the thrown rocks are disappearing.

[EDIT: confirmed it for large rocks thrown by troll brutes as well.]