Security question only after logout

Only after I logout and login again I get the security question “Show me you are not a machine: 4+8=”.
a) resolving the equation is not needed, if I leave it empty I can login, why ?
b) The security question is not always shown, always after a login/logout. Never after a a browser quit/activate. Is this normal ? Can it always be “not shown”
Settings:
– custom redirect after login
– custom redirect after logout, but also the standard redirect had this behaviour
Thanks Marcel

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