There is a bug in agenda syncing on different platforms. I have an account that i use for a dog grooming note cards. I have some things i do on a mac computer and other edits on a mac laptop. somehow periodicaly the laptop will not snyc to the agenda on my Mac vis versa, even wehn i force close on both devices. it just apperares hours after i actually needed to use it.
Yeah! Here is the same.
I found this on stack exchange.
Unfortunately, it seems iCloud has been struggling a bit lately, perhaps due to the new OS release.
Trying what BruceN suggests is worth a try.
Something that can help sometimes is signing out of iCloud in Settings app, and signing back in again. Doing that on each device.