Subscription calendar URL not showing events in Calendar app

I have a URL for a subscription calendar which has no errors according to iCalendar Validation.

When I use this URL for a subscription to Calendar it doesn’t show any events! However, when I use the URL to create a new Calendar in Google Calendar it works as expected. I can then use the “Public address in iCal format” (for which Google says: Use this address to access this calendar from other applications.) to create a subscription in Calendar which works as expected!

Why doesn’t the original subscription URL work in Calendar?

John

MacBook Pro Sequoia 15.7.2