I have two tiles using a Google secret iCal address. They are working fine when I am logged in as the primary user, but they are blank when logged in as the shared user. The shared user has Interact privileges.
Primary User
Shared User
Thanks for posting and sorry to hear that things aren’t working as expected!
Would you mind sending a PM to @support with the dashboard ID so we can take a closer look? You can send over the URL to the dashboard and we can grab the ID from the end of it. 
Thanks for sending the PM! We tracked down a bug with calendar sharing that snuck its way in recently and have pushed out a hotfix which should resolve the issue. 
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