If I'm playing in a world created in Minecraft 1.20.1 or prior that was upgraded to 1.20.4, and I take a piece of paper and name it "SecretPassword," then that item _will not _stack with paper named SecretPassword created on prior versions. This effectively breaks many redstone contraptions as well as datapacks, requiring all filter items to be discarded and replaced with newly renamed items.

The root cause is that, in 1.20.1 and prior, items named via anvil would have their NBT represented as

{display:{Name:'{"text":"SecretPassword"}'}}

whereas in the new version the NBT data is simply:

{display:{Name:'"SecretPassword"'}}

Note that while this is also true for named block entities (chests, hoppers, brewing stands), the process of upgrading the world seems to have converted the names successfully to the new format.