when my players craft the diamont block obtains the city protector, why?????

You probably have meta-autoset to true. Set that to false and it will only count as City Protectors the same block material with name and lore!

And remember we are now using this github page: https://github.com/elBukkit/PreciousStones

There you can report issues and also download the latest stable version for PreciousStones (for mc 1.15.2)