when using the Backpack free add-on, a few times i have placed the backpack on the ground and it totally disappears with all my things

When using the Block Factory BackPacks add-on, when placing the backpack on the ground, sometimes it disappears with all contents. So far have not reappeared. 

This will be distressing if this is used in a survival world with no cheats.

Unable to contact Block Factory directly as the marketplace has no option to contact them

 

Update - back pack and contents have reappeared, but not where i placed it and on a surface that was not there when it disappeared.