I am testing out the Minecraft beta 1.12.0.17 on Xbox one the only issue I have found so far is the Llama herder you see in the game. For about three to four minutes you have seen the Llama's but not the herder you saw them being walked with an invisible character with bubbles floating then after a while he appears.
MCPE-52879
Llama herder not being seen for a few minutes
Resolved
Cannot Reproduce
0
Sometimes, the traveling merchants drink potions of invisibility when a mob gets close by. The bubbles you saw might have been the particles from him drinking the potion, which also explains why he re-appeared after awhile.