I'm not sure if this will help you, but I will share with you in case it does.
In summary:
Apparently, the iCloud email account has to be turned on in the Mail app on the iPad Air for the Calendar to sync with iCloud!
Why? I don't know.
What happened:
I recently lost all entries in the Calendar on my iPad Air.
Checkled all settings for iCloud and Calendar on the iPad Air. All iCloud selections were turned on.
Checked online forums. No solution found.
Then I remembered:
Last week, I removed my iCloud email account from my Mail function to keep things tidy, since I do not use the iCloud email account. Instead, I use the email service from the local ISP.
After that, I did not use Calendar function on iPad Air for a few days, therefore did not realize that this caused all Calendar entries to disappear.
(Calendar works ofine n my iPhone.)
I have just added my iCloud email Account back into the mail afunction. Right away, all iCloud Calendar entries showed up.
Hope this helps!
Cheers!