Kerberos/SPN issues with multiple instances using dedicated IP's/Ports

Hey all,

Here's the setup.

Server A
- Instance1
- Instance2
- Instance3

Server B
- Instance1
- Instance2
- Instance3

Each instance has a dedicated IP + port that is set to 1433. I can connect to each instance by name and by IP without issue.

I then have a DNS alias in front of each IP.
- From Server A, I can connect to all 3 aliases for Server B
- From Server A, I can NOT connect to any of the instances for Server A
- The same goes for the other way around

When connecting, I get the is from an untrusted domain. I check the error log and I'm getting the SSPI handshake error. 

Checking auth_scheme from sys.dm_exec_connections, shows all my connections coming in under 

Each instance has an SPN, with no port specified. I've manually added a new entry for port 1433, and still the same issue persists.

Help would be most appreciated.

Thank you!