Im not sure I am reading this correctly, let me jump in game quickly and I will come back with my understanding.

[edit=1]
I couldn't do what I wanted to do in game so let me post what I think you are saying and hope that it is what you mean.

You place the plastic in the craft window, and realise you need another part, and drop the plastic pieces you had on the ground - likely because you needed the four empty spaces.

Getting the item you wanted you put the new metal item on the craft menu and craft it.

My question to you then, is did the craft process the item and remove the plastic from the game world?