1.13 introduced a new feature which basically allows to get different blockfacing variations of mushroom blocks by placing them next to each others. Here is a demo: https://www.reddit.com/r/Minecraft/comments/7eu5em/how_to_change_your_mushroom_blocks_to_the_porous/
I would like to disable this feature so that I can use these different variations to create new blocks but there is no event for that. So here is what I have to do to fix it (and this is still not perfect, it has many bugs): https://github.com/Th0rgal/Oraxen/blob/master/src/main/java/io/th0rgal/oraxen/mechanics/provided/block/BlockMechanicsListener.java

This technique is the only one that allows to add new blocks correctly to the game and it would be really great if we could disable this very annoying mechanism directly from the game (the slightly buggy techniques that we have to use to get around this problem are very resource-intensive), otherwise an event that would be called (and that we could cancel) every time the blockfacing of a mushroom block is updated would be just as great !