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Avoid glitched blocks

Glitched blocks in some areas/custom worlds

Glitched blocks

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.
Open config.yml and set this option:
config.yml
fix-glitched-blocks:
enabled: true
only-new-chunks: false

Another solution

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 because they don't naturally generate around the vanilla world.

Optional settings on Purpur

This only works on Purpur. Spigot and Paper don't have this feature.
Enable these options in purpur.yml configuration:
Note: disable-chorus-plant-updates: true will completely stop chorus plants from updating. So you won't have the chain break effect on them when the first block is broken.
Example:
blocks:
disable-mushroom-updates: true
disable-note-block-updates: true
disable-chorus-plant-updates: true