Alex, are you running the latest version of iOS 13.x?  We have had other reports of contacts syncing with iOS 13.x, and we're looking into the cause and a fix.

Yep, I'm on iOS 13.3.

Hello everyone,
I would just like to update this thread so everyone coming to this thread is up to date as to where we are.

Unfortunately, we have been unable to replicate the issues seen here. We have tried multiple iPhone and iOS devices and have been able to successfully sync over contacts in every case, including when given credentials to one of the users having this issue so that we could try to sync it from their server.

This is not to say that we don't see this as an issue. We are still actively looking into ways in which we may fix or replicate this. That being said, the newest versions of iOS (13+) have been plagued with issues related to EAS and mail in general and this could very well be an iOS upgrade issue since we've only had reports of it on iPhones (v13.3) that we have been able to confirm are still an issue after removing the EAS connection under the connected devices. Apple has released a new beta iOS that includes some more bug fixes. We will continue to try and keep this thread updated as we get any new information.