Singletons (mainly Air#INSTANCE) and constants are incompatible with BlockSpec#maybeWaterlogged

[NotStirred]

BlockSpec#maybeWaterlogged modifies the returned object; if that object is a constant any previous or future reference to that object is modified.
For example if a world contains air and air + waterlogged=true then all air blocks in the entire world will be waterlogged.

Any BlockProvider implementation that returns the same object from getBlockByTag for different arguments could run into this issue.