Found a interesting softlock. Just went into this side area, and when I came back out Poppin was missing. Switching showed him on top on the map somehow. I can run around and fall off, but respawning does not set Poppin back in bounds, and so I can't get Poppin to leave the cave to push the bolder. 
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.