Cannot set iCloud as default account for contacts

I am following all the suggestions on Get help using iCloud Contacts - Apple Support but my iOS settings won't allow me to select iCloud as my default account for contacts. iCloud was my default account in the past. I disabled and re-enabled iCloud it to solve some synchronisation issues. And now my iPhone allows me to select my Exchange account from work and "On My iPhone", but not iCloud. Why?

And how do I solve it?

This is the advice on Get help using iCloud Contacts - Apple Support:

If your device uses multiple accounts like iCloud, Gmail, and Yahoo, make sure that iCloud is your default Contacts account:

Tap Settings > Mail, Contacts, Calendars.

In the Contacts section, tap Default Account.

Make sure iCloud is selected.

These are my iCloud settings, proving that I have one and Contacts are enabled:

And these are my Contacts Default Account options, proving that my iPhone won't allow me to select iCloud as my default account for Contacts: