Ok I could reproduce the bug by modifing your source code. I created TSubclassOf WeaponsClass; attribute in Character Class. Than in the editor I created derived BP_Weapon class from AWeapon class.
This BP class I set as WeaponsClass in Character properties.
I attached archive that includes c++ source files and only modified BP classes (4.12 git version). So you need to unpack content only in your myproject12 folder.
To reproduce bug you need only to select the character actor for sequence record. You will see weapon mesh than at root position.
When adding weapon to record too you will see additional bug. There is weapon mesh at root position and weapon actor high above that head of character that was not snapped to socket.