I recently noticed while building an issue regarding red_mushrooms in the end. I placed 7 mushrooms on my build made of wool within the end and over time the enderman slowly duplicated them into stacks and stacks of mushrooms. And since the enderman kept placing them it eventually covered most of the main end island. This also due to the enderman not despawning while holding an item, caused natural spawning for enderman to break until all the enderman holding mushrooms were all killed. It's not a fast process, as I was in the end for hours before the mushrooms duplicated to extreme levels. The enderman also have a tendency to pick up red_mushrooms over the other blocks in the end which makes the duplicating process faster. Also, the game was on hard mode during this time.

Steps to cause bug are as follows.

1. place at least 1-10 red_mushrooms in the end (preferably placed on wool first) 
2. afk with the mushrooms loaded in your chunks for at least 2+ hours