Cached Webcal Subscriptions are only updated every few days


I’ve recently updated my Nextcloud installation to version 15 which now supports caching Webcal Subscription and exposing them to clients which don’t support Webcal Subscriptions over CalDAV yet.
After adding a few Webcal Subscriptions to my calendar, the events from those calendars are visible after some time. But even if the TTL/Refresh-Interval of the Subscription is set to 1 hour, the calendars are only refreshed every few days.

I can see requests made by Nextcloud using the “Nextcloud Webcal Crawler” user agent every few days in the access logs of a server hosting one of the subscribed calendars. Shouldn’t those requests be made every hour if the refresh interval is set two 1 hour?
I took a look into the Nextcloud database into the oc_calendarsubscriptions table to confirm that Nextcloud correctly recognized the refresh interval: All of them have “PT1H” as refreshinterval which means they should be updated every hour.

I’m not sure what exactly triggers the update, I guess it’s done by the usual Nextcloud Cronjob. In my case it is set using Crontab to run every 15 minutes.

Am I missing something?