I have 6 sites where MainWP claims that the site health needs to be improved, but when I check the site overview on the sites, there are no security issues detected. I can see that another member had the same issue back in 2021: Site Health information.
Site Health that MainWP is reporting on the Sites > Monitoring page and Security Issues that can be found in the Child Site Overview > Security Scan tab are unrelated.
Site Health data is generated by a WordPress core functionality, and MainWP is merely transmitting that data.
Can you check on the child site itself, on the WordPress Dashboard > Tools > Site Health page, if the data there corresponds to the data present on the MainWP Dashboard?