Azure AD Connect - User moved in local OU (not synced) is getting deleted in Office 365

Hello!

I have recently used Azure AD Connect tool to sync Office 365 users to local AD. I need to have a user as "cloud-only", so I moved them to an additional OU which ommited from syncing.

After next sync, the user shown in the Deleted Users in Office 365 AD, and I restored the user, which now shows as "in cloud".

After next sync, the user is moved to the Deleted Users again.

What do I have to do to keep the user as cloud only and from falling into the Deleted Users after every sync?

Best regards

K