Why does the wrong username show in my commit messages?
firstname.lastname@example.org, you need to make sure your repository configuration is set for that username. Also, make sure you have set your global username/email address configuration to a validated email address.
If the global default is not configured or if you have not validated your email address, the committer appears as unknown for your Bitbucket activities. Also, if you have multiple Bitbucket accounts, you may mistakenly commit your code under an email address that maps to an account name you did not intend. To have the existing commits on a repository map to a different account, you can use Username aliases for the repository in question. Aliases are per-repository. To set up an alias for a repository, you must have admin rights on it.
For information about configuring your username or aliases, see Set your username for Bitbucket Cloud actions.