I had the same issue. All my 3rd party iMessage apps showed a logo and nothing else when I would swipe to each one of them in the lower window area.
After reading this thread, and watching some videos on how they're supposed to work, I messed around with them and found that they're essentially not working.
After toggling them on and off, I eventually I found that I had to delete the normal app on my deivice and reinstall it. Once I did that and went back to the app in iMessage, one by one they would start working again.
So it looks like there's some type of bug with the 3rd party apps and the way they're loading in iMessage.
I've noticed a few other bugs with iMessage when I rotate from portrait to landscape.
None of this is surprising as iOS 10 is so new and iMessage received a massive over haul.