The Bug
Enchanting rare items such as a command block or knowledge book doesn't change the text color to aqua but keeps it as purple
Reproduce
Go into creative
Do /give @s knowledge_book
Get an anvil and enchanted book
Enchant the knowledge book
Observed Result
The text color of the enchanted knowledge book is still purple
Expected Result
The text color would become aqua
I was able to confirm. I've updated your report since this affects all rare items