Alright so i decided to send out a settler to explore and gave him 10 stimpacks and 10 radx's. Right after i assigned him to explore and right after he started running for the door, a fire broke out. After the fire was put out by the dwellers in the room, my dweller went back to where he was before i had assigned him to explore. I repeated the process to send my dweller to explore but to my surprise all 10 stimpacks and radx's that i had given the dweller had disappeared ...

When dwellers return, their Stimpak/Radaway inventory is automatically transferred to your Stimpak/Radaway storage. Every time you send out a dweller, their Stimpak/Radayway inventory is empty and you have to equip them anew.

I'm guessing that because your dweller never actually made it out, the Stimpacks/Radaways were not actually assigned to his inventory yet but remained in your storage. In that case, just equip him again. If however you are saying that the Stimpacks/Radaways completely disappeared from your actual storage and you weren't able to equip the dweller for a journey, then that's probably a bug.