Hello!
This issue is due to the GCP managing its own SSH key file, authrized_keys.
Because we created the instance, it's likely to be entirely under our control.
These instances are all controlled by GCP functions.
The authorized_keys file is one of them, and to fully add it to the list of public keys here, you'll be able to:
You should go to Metadata - SSH Key - Modify from the left menu and add the public key as shown in the lecture note.
This adds its public key to all instances, eliminating the need to add authorized_keys each time.