Calendar and iCloud not syncing correctly

Some of my icloud calendars sync between Mac, iPhone and iCloud and others do not. My iPhone syncs correctly with my Mac and vice versa, but iCloud only syncs certain elements. None of my subscription calendars sync with iCloud and my main calendar (which is 100% an iCloud/Apple calendar does not sync). I have an error on my Mac saying the calendar can not be found on the server, yet it syncs perfectly with my iPhone - just not iCloud.

This must be something to do with iCloud, as of today (18th August 2025).

I have changed no account settings, passwords etc and the Apple ID has been in place since 2005. I have never encountered this before.

My set up is:

16" MBP M4 Max, 128GB RAM, 4TB SSD

Sequoia 15.6 I use Safari as my main browser, so shouldn't have iCloud issues

Good, solid and functional fast internet connection of 100GB+

iPhone 14 Pro iOS 18.6.1 (latest as of today)

Any help much appreciated!