According to documentation of applyToCrNames:
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Apply this security config to the brokers defined by the named CRs in the current namespace. A value of * or empty string means applying to all brokers.
Default value: All brokers defined by CRs in the current namespace.
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However, not having applyToCrNames in yaml still applies security settings to brokers in other namespaces, not just the current namespace.

To replicate, define ActiveMQArtemisSecurity setting in one namespace without applyToCrNames (therefore the default value will be used). You will see that this security setting is applied to all brokers in other namespaces that do not have their own ActiveMQArtemisSecurity instance with applyToCrNames.