This is only a graphical glitch, this state won't cause duplication bugs or similar.
Fix on Spigot/Paper
This is normal if you use REAL type to create custom blocks.
ItemsAdder uses mushroom blocks to create them.
ItemsAdder just checks some biomes to fix the glitched blocks to avoid using too much CPU.
This fix works only for REAL blocks (mushrooms).
If you want to apply the fix to any biome you have to set config.yml like that:
Click here to expand
This will basically enable the fix on each of the Minecraft overworld biomes.
Feel free to remove the ones you think that won't have any mushroom spawned, to avoid high CPU usage.
Another solution is to avoid using REAL custom blocks type (mushroom) and use REAL_NOTE custom blocks type.
REAL_NOTE uses Noteblocks to create custom blocks, so you won't have this issue.
