On my realm we have a few issues. Sometimes players will be disconnected from the game saying they have been "kicked" from the realm. When they log back into the Realm they notice that "phantom" blocks have appeared around them in random locations. 

When I go into creative the "phantom" blocks are actually Barrier blocks. 

Sometimes players get kicked over and over again even when they're the only ones who are online.

Barriers seem to spawn every time someone is disconnected. 

There are not any command blocks that are causing it. My guess is it is caused by a hacker. Shortly before it started there was an 'Minecraft:npc' that spawned that was i able to remove with /kill npc command. 

Before killing I downloaded the world and messed with it in an offline world. When you interacted with it, the npc would spam some command and crash the world over and over without being able to join back. I reverted the world and cleared the npc then this started happening. The hacker was since removed before the above issues.