This bug does not only affect the first player that enters the game. After about two weeks of running a busy server, the entity IDs will overflow, which results in no player being able to use fireworks. Negative entity IDs are also ignored by this function (from 1.12.1 decompiled with MCP):
public boolean isAttachedToEntity() {
return ((Integer)this.dataManager.get(BOOSTED_ENTITY_ID)).intValue() > 0;
}
What is this nonsense, and why did this get 'fixed'? The comparator never worked this way. If you put buttons/levers/torches anything next to it, it dosn't turn off. Only redstone dust could be used for the compare input. Why should it be different with redstone blocks? I think actually "fixing" this bug is a far worse bug, this should get reverted!
This also affects the search item tab in the creative inventory screen, randomly writing an 'e' into the search bar when opening it.