The Clue Finders' 4th Grade Adventures

Joni's backpack is different in this game than in others. Instead of opening with a vertical zipper on the back, it has a latch on the bottom and opens similar to a suitcase.

When the ClueFinders transform into the Egyptian gods when entering the Ancient Pyramid, Joni's backpack disappears with no explanation given and it never appears again after she and her group transform back into their normal selves at the end of their adventure.

Followed by The Clue Finders' 5th Grade Adventures: Secret of the Living Volcano (1999)