Anyways, that shouldn't be the correct behavior. After updating to 4.1.0 (and above), you have to unlock the wallet to create a new one and the newly created wallet has the same password as the wallet that you've unlocked. However, this shouldn't convert the passwords of any other wallets.
I confirm this.
I created wallet A and wallet B with the version 4.0.9 and then updated electrum to 4.1.4. I unlocked wallet A and created wallet C.
There was no change in the password of wallet B.
If you go to Settings > Password, you should be able to see "Change your password for this wallet". This means that the password change only effects the current wallet.
I changed the password of wallet C using "Change your password for this wallet". Now I have three wallets with three different passwords.
I cleared electrum's data and created two new wallets (with 4.1.4)
I went to "setting" to check if it's possible to change the password of one the wallets without the other one being affected.
There was no "Change your password for this wallet". Instead, I have "Change your password" option.
Conclusion:
If all wallets have been created with the new version (4.1.0 and after that), there is no way to have different passwords for different wallets.
If you have already wallets with different passwords, even wallets created with 4.1.0 and after that can have unique passwords. (To do so, you must create a new wallet with a password same as one of existing wallets and then go to setting and change its password).
Let's say I create wallets A and B with 4.0.9. I update electrum to 4.1.4 and create wallets C and D. It's possible to have four different passwords for these four wallets.