Contacts sync to other phone although we are using different iCloud iD

Hello Asab8,

Thank you for posting in the Apple Support Communities. We understand that contacts are unexpectedly syncing to another device, and we'd like to help.

For this issue, consider whether either device had any shared account in the past. For instance, if both devices used the same email account, and those contacts were linked to that account, then it could be likely that those contacts saved to the device that had iCloud Contacts enabled.

We also suggest considering whether both devices sync content to the same computer. If so, this could also sync data if the same Apple ID is unexpectedly signed in to iTunes or the user account. Learn more about syncing via a computer here:

Sync your iPhone, iPad, or iPod using your computer

To delete unwanted contacts, we recommend taking these steps:

Manage and delete contacts on your iPhone, iPad, or iPod touch

On the other hand, if no accounts were shared between the two devices, we recommend that you contact Apple Support directly for further assistance. Depending on your region, you may be able to reach them here: 

Get Support

We hope this information is useful. Kind regards.

Asab8,

Thanks for following up with us. We’re glad to see that you were able to trace back to how those contacts synced between the two devices.

Have a great day.