Hello oo Jesse oo,
If your bag is full when you are doing treasure hunter you cannot claim it to go into your inventory. The only option is the bank or to exchange it for oddments. My assumption is you accidentally hit the exchange for oddments or when the day reset your reward might have gone with it.
Under Runescape support, you can always submit a lost item claim and allow for them to process it if you think this has been done in error.
21-Apr-2020 00:35:08