Consider following script (correct here name of alias - i used 'employee'):
===
    connect 'localhost:employee' user 'SYSDBA' password 'masterkey';
    create user foo password '123';
    create user bar password '456';
    commit;
    create role worker;
    create role manager;
    commit;

    grant worker to foo;
    grant manager to bar;
    commit;

    --connect 'localhost:employee' user 'BAR' password '456';
    connect 'localhost:employee' user 'BAR' password '456' role manager;

    set list on;
    select rdb$get_context('SYSTEM', 'DB_NAME') as where_i_am, mon$user who_am_i, mon$role whats_my_rtole
    from mon$attachments
    where mon$attachment_id = current_connection;

    set term ^;
    execute block returns(where_i_am varchar(255), who_am_i varchar(31), whats_my_role varchar(31)) as
        declare v_sttm varchar(2048);
        declare v_user varchar(31) = 'foo';
        declare v_pswd varchar(31) = '123';
        declare v_role varchar(31) = 'WORKER';
        declare v_extd varchar(255);
    begin
        v_extd = 'localhost:' || rdb$get_context('SYSTEM', 'DB_NAME');
        v_sttm = 'select rdb$get_context(''SYSTEM'', ''DB_NAME''), mon$user, mon$role from mon$attachments where mon$attachment_id = current_connection';
        
        execute statement
            v_sttm
            on external v_extd
            as user v_user password v_pswd role v_role
        into
            where_i_am, who_am_i, whats_my_role;
        
        suspend;
 
    end
    ^
    set term ;^
    commit;

    connect 'localhost:employee' user 'SYSDBA' password 'masterkey';
    drop user foo;
    drop user bar;
    commit;
    revoke all on all from foo;
    revoke all on all from bar;
    drop role worker;
    drop role manager;
    commit;
    show version;
===

On 2.5.9.27126 out will be as expected:

WHERE_I_AM C:\MIX\FIREBIRD\FB25\EXAMPLES\EMPBUILD\EMPLOYEE.FDB
WHO_AM_I BAR
WHATS_MY_RTOLE MANAGER

WHERE_I_AM C:\MIX\FIREBIRD\FB25\EXAMPLES\EMPBUILD\EMPLOYEE.FDB
WHO_AM_I FOO
WHATS_MY_ROLE WORKER


On 3.0.5.33086 and 4.0.0.1378:

WHERE_I_AM C:\MIX\firebird\fb40\examples\empbuild\employee.fdb
WHO_AM_I BAR
WHATS_MY_RTOLE MANAGER

WHERE_I_AM C:\MIX\FIREBIRD\FB40\EXAMPLES\EMPBUILD\EMPLOYEE.FDB
WHO_AM_I FOO
WHATS_MY_ROLE NONE <<<<<<<<<<<<<<<<<<<<<<<<<<< [ Why ? ] <<<<<