Single User Can't Access RDP by DNS, only by IP

I have three domain controllers, dc01.ad.exmaple.com (10.83.78.10), dc02.ad.example.com (10.23.20.15) and dc03.ad.example.com (10.24.20.15). They are all in the standard Domain Controllers OU.
dc01 is a physical machine running Windows Server 2019. dc02 and dc03 are Azure Virtual Machines running Windows Server 2022, dc02 in East, dc03 in East 2.
All the machines are on a VPN and replication is working.

I also have three users in the Domain Admins group: user1.admin, user2.admin and user3.admin.

user1.admin can not connect to RDP on dc02 by name, only by IP address. When connecting by hostname, the user gets the message "The logon attempt failed" and nothing is logged anywhere that I could find in Event Viewer. The other users can connect to RDP with no problem using the hostname. If user1.admin tries to connect with the IP address instead of the hostname, everything works fine.

Where else can I look?