According to this thread, the problem of "Goblin-Hide Sack" spontaneously changing into a box with no special properties is said to have been corrected.

Just an FYI, it still happens. It seems to occur if the bag is dropped due to death.

Also, the Lizardhide sacks from the Shadow Tribe merchant lose their weight property if dropped due to death.

The Rat Leather Bags of the Kuo-Tao work fine, and perhaps can be used as a base comparison to deduce what the problem is?

The goblin-hide sacks that still have this problem are probably old ones that we thought we'd pulled out of circulation, but people come and go and might be carrying bugged items without even knowing it, and miss announcements about needing to exchange items...

Whatever we did for the goblinhide sacks will have to be done for the lizardhide sacks. Bug fixes concerning items are generally not retroactive, and will require us finding everyone with a bugged item and replacing it. We generally don't waste lines of code to do this automatically, because it happens more often than we like to admit and eventually it would add a lot of overhead.

AScottBay wrote:

"The goblin-hide sacks that still have this problem are probably old ones that we thought we'd pulled out of circulation, but people come and go and might be carrying bugged items without even knowing it"

I picked up a goblin-hide sack fresh within the last 2 weeks and it did this. Unfortunately, I can't recall if the sack was in the "remains" of a slain enemy or in a quest container. But I do recall thinking at the time that it shouldn't have the bug because it was a fresh drop. When I died, though, it morphed into the box sans weight property.