Why do calendar notifications bring macOS Calendar application to the foreground?
So as it happens, I had recently turned on a feature in macOS Mail (mail.app) called "Add invitations to Calendar automatically". Disabling this feature immediately fixed this issue and does not prevent Calendar from receiving/notifying/displaying calendar invites. In honesty, aside from removing this really annoying side-effect it doesn't seem to have any downsides for my usage of Mail/Calendar.

Fix: Mail > Preferences > General > Disable "Add invitations to Calendar automatically"