Hello,
So I have no idea why that is, but for some reason Vault is overwriting that and there is absolutely NO option to disable that. I am very sure that it is Vault, because as soon as I uninstalled it, the prefix magically went back to normal. Now the problem is, I kinda depend on Vault as I have not found a better alternative to it.

I am using Paper 1.12.2 version 1620

I have no idea why and how this is caused. Any way to fix this besides uninstalling Vault?

Just realized this is not the official one, sorry.