Hello.
The problem is verified on both Linux and Windows systems.
When you install Firebird 3.0.9 with checked "Generate client library as GDS32.DLL for legacy InterBase support?" and you set your own password for SYSDBA user, after the installation you can log in as SYSDBA with changed password AND with masterkey as well.
SRP user SYSDBA has changed password, but Legacy_Auth user SYSDBA has masterkey password.
In the result, although we changed SYSDBA user's password while installation, Firebird still accepts SYSDBA logins with masterkey password.