That's a new one, not sure how Poppin ended up there. Were you split up when you entered while this happened?
Viewing post in Poppin & Jupa: Pocket Adventure - Demo comments
I didn't enter the cave split up, but I did spilt up to enter into the area with the chest, as Jupa is the only one who can crouch. To note is that Poppin was exactly where I left him horizontally, he was just transposed vertically to the top somehow. I Jumped down to the area in the screenshot to get a better image. Also he was off screen when whatever sent him to the top happened.