Edit: After updating my devices to 26.4.1 (were on 26.4) the iCloud Sync still did not work. My problem was the container not being recognized. I tried to troubleshoot all I could to no avail (Xcode entitlements, plist, Capabilities, CloudKit console, etc). As a last resort I created a new container for my project and that was my solution. I can only imagine the handshake was corrupted somewhere.