For reference, I see this on page 7 of the rules:
No more fighting cards in the Robinson stack
During the whole game, the following rules are active:
If you need or want to draw one or more fighting cards from the Robinson stack, but there are not enough cards left to draw, you draw the remaining cards until the stack is depleted. Afterwards you must take the topmost face-down aging card from the aging stack and shuffle it into the fighting cards (Do not look at the aging card!). Place all cards back on the storage board as the new Robinson stack and draw the missing cards.
You only shuffle the Robinson discard pile, if you need to draw another fighting card (not already at the time, when you draw the last remaining card of the Robinson stack).