I will preface this with version 1.12.0 bedrock is absolutely the worst buggiest update ever. Period.  You supposedly fixed a bunch of  issues that I never had and now not only are water mechanics and Mob's acting up left and right we also have mobs disappearing over many peoples platforms. This has nothing to do with https://bugs.mojang.com/browse/MCPE-47922 so dont redirect or consider "resolved" or duplicate. Because you guys keep closing the actual issue stating people are out of range or other stupid excuses. Consider it! Dont shut everyone down. 

My individual scenario has multiple issues. Pre 1.12 i had a endermen farm in the end. Worked great for 6months on the realm i purchased from ingame. "Actual minecraft realm." Playing today and another member and i are wailing out on endermen 20blocks STRAIGHT below the mite (ofcoarse hes named(its been there for 6months). Other player goes up the ladder and the endermen stop coming down i go up to look what happened and the end mite is gone. Player left the end, so he didnt touch it. Im realm owner and other player is also, we are only players and neither of us touched anythin NO EXCUSES NO EXCEPTIONS!! MITE DESPAWNED OR DISSAPEARED. This issnt the first time. I had two mites running in the end and in 1.12 the second mite wont even last 10min before disappearing!!! (again with tags) the mite that survived through the update 6 months old stayed around till today. Now he keeps disappearing!! NOT ACCEPTABLE.!! People keep reporting similar issues weekly dont brush it off let it get voted. It seems to be happening to mobs people are trying to keep... and less to random mobs. Look into that!