TPM auto-unlock still relies on measured boot integrity (Secure Boot/PCRs), so it protects against offline theft and tampering when the machine is off or storage is removed.
But if an attacker has repeated physical access during boot, the protection depends on whether you’ve added extra factors like a TPM PIN or pre-boot passphrase. Login prompts don’t re-protect the disk once it’s decrypted.
In practice, for my use case (mostly shutdown or battery-dead scenarios), this is an acceptable trade-off for convenience. If your threat model includes targeted physical access during boot, then keeping a pre-boot secret is still the safer choice.