In 1.12 I had made a zombie spawner into a drowned farm by way of water logged cobweb in a drop shoot with a trident killer on the bottom to produce xp.  Since the 1.13 update I've been having instances where mini drown spawn right next to me and sometimes in multiples. Once in a while they appear as zombies and then change a second later so I believe something is happening to mini zombies when they convert that makes them behave more like a normal spawn, spawning in multiple and on the nearest solid block (which would be where I afk.  I've had 7-8 of them attack me at once though.  Could be duplication or just a coincidence of having more than one mini zombie convert simultaneously.  But now I can't afk at my farm or risk death.