Hmm… good catch. And I do agree that this should be expected behavior.
From a technical POV, the plugin will only reevaluate group membership on two different conditions:
- When the boolean expression in the group settings is saved
- If a user is added or removed for a group mentioned in the boolean expression.
When a new user registers, neither is the case.
Can you try to change the expression to trust_level_0 AND NOT anonymous_users and report back if that resolves the issue?