TYPO3 Core

This is not a bug necessarily. More a design flaw.

Go to: System -> Reactions and click on the password generator button on the right-hand side of the input field for the "secret".
TYPO3 generates a random secret and disables the input field.

If backend users realize at this stage that the suggested secret does not meet their requirements, for example too long, too short, too cryptic, no special characters, etc. they don't have an option to switch back to manually enter a secret.

I understand that they can either discard the current configuration without saving it, and start again -- or to save the current configuration incl. the randomly generated secret and change it later by re-opening and editing the record.