My iCloud calendars on iOS haven’t synced since day one of iOS 11.
I’ve spent a lot of time with support, seniors, sending in system reports to the engineers etc.
Signing out and in did absolutely nothing for me. We tried a lot of things. iCloud Calendars gone on iPhone and not syncing from iPad either, but showed up there.
On Mac Calendars syncs flawless with iCloud though.
I gave up with Apple support, didn’t lead anywhere, I was just working for free to hand over systemreports to the engineers. So I started to use my Synology NAS to sync calendars instead.
Been flawess ever since. But when I beta tested iOS 12 they disappear too, and some lost iCloud calendars started to show up
Ah well, seems like they’re working with it. I went back to 11.4.1 and my NAS-calendars came back, peace restored