Further questions on the mechanics of treasure hunter are probably best put on the appropriate board, but seeing as I'm here I'll answer this one.
The multiplier doesn't always apply - it's random. You'll know if it's applicable as the coloured boxes at the bottom will be lit up
Here's an example. Somewhow, I've managed to unlock all the multipliers since they last reset this promo. You'll see the "Three" multiplier is lit. My next key will use that multiplier. The next key may use any of them from 1 to 7.