The quarantine role is a role with no permissions that cannot see your channels. Defendr adds it to a member the join gate stops and remembers it, so the role comes back if the member leaves and returns within 30 days. Defendr's own role has to sit above the quarantine role, otherwise Discord refuses to assign it and Defendr falls back to a timeout.
Example
You drag the Defendr role above the Quarantine role and the Security Score issue disappears.