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Your keepkey behaves as designed.
This passphrase function was initially introduced by the Trezor team to protect you from a $5 “wrench” attack:
https://imgs.xkcd.com/comics/security.png
Adding a passphrase to your seed is like creating a 25 words seed. The keys will be different between a 24 words seed and a 25 words seed.
The same will occur if you enter an additional word (passphrase) to Trezor.
The passphrase function on both Trezor and Keepkey is only if you want to create hidden accounts in your hardware wallet: https://doc.satoshilabs.com/trezor-user/advanced_settings.html