I did some background check and your configurations are perfect. However, the reason for you to get both the alerts are different.
I believe you had Marked maintenance during that period and this problem occurred just after the maintenance. After the maintenance window was over the first poll happened for switch-monitor whereas for the internet-line monitor the poll happened after that (this just based on the polling frequency configured).
Hence while alerting for switch-monitor we weren't aware of the internet-line monitor was down since it is maintenance (which is a separate status). If the current status of the parent monitor is in maintenance that won't be accounted as Down while suppressing the alert for the dependent.
In a normal scenario you wouldn't get the alert for switch-monitor. We've taken this use case and will try to address it in the product.
If you have a support ticket open please reach out to us and we can give you the complete analysis.
Product Manager, Site24x7