Not sure I understand this. If someone finds my 12 words but not my passphrase, he can see I have a smaller balance than the reality? I have 10 BTC, and without the password, someone sees I only have 0.1 BTC?
You can use
IanColeman.io to try it (of course only do this online to see how it works, not to create private keys you're going to use):
12 words: plunge snow gorilla wool help income mom kitten volcano warm top girl
First address: 15Q9XKiYX4RZhNLdKYt894o1zguyhufoPZ
Now add BIP39 Passphrase: \q?mpN[!\4`LA5w*
First address: 19mYSjt1VaiGpTop31uEMWJFYbW76MCRJ3
Without the password, the same 12 words lead to different addresses.