So something that I've had to deal with, while using this plugin is basically "preserve-attributes" not behaving the way it should
The way I fixed this was by editing the source code of tf2items_manager to set the default value of "preserve-attributes" to be 1, or on.
What this does is it tells the game that unless specified, the game will preserve the attributes of the weapon.
I'll attach a download for the new tf2items_manager.smx for anyone who had the same problem, maybe it will work for you.
NOTE: If you want to preserve the attributes of the weapon, do not include "preserve-attributes" "1" on the weapons you wish to preserve the attributes of.