Yes, if you deconstruct an item that you have/had in your Material Inventory, you will only get scrap.
However..
Lets say you have 100 [Premium Lustrous Synth Bonded Attachment] in your Material Inventory that you want to decontruct and get the right materials:
- Craft a new [Premium Lustrous Synth Bonded Attachment] that does not have been in your Material Inventory.
- Take the 100 [Premium Lustrous Synth Bonded Attachment] out of your Material Inventory and merge them with the new one you just made.
This worked for me, and it should work for you.