My question is this. Is the older server using a different version of SMB?
Yes
Win 2008 Server Uses SMB 2.1
Win 2012 Server Uses SMB 3.0
Is it possible that I hit some kind of bug with the Terastation's SMB implementation?
Maybe. I don't know. I forget what versions of SMB the Terastations support out of the box. Something doesn't play nice between Buffalo and MS in my experience.
I can't say for Windows 2012 and AD, but I do know with Windows 10, these Terastation 3400 and 5400 series NAS devices, they don't like having multiple connections with the same account- with different machines. Why? I have no idea.
I'm not using AD with my units, I use an account created on the terastation itself. After rebooting a Windows 10 PC, I wind up running my drive map scripts sometimes 3 or 4 times, before they will map. The scripts will often throw a 'Multiple connections to a server or shared resource by the same user' error. Why? I have no idea- as they are authenticating with the Buffalo/Linux account- not a MS account. I shouldn't be seeing such an error.
As to your case:
I assume you're authenticating with a Windows AD account to map these shares on the servers themselves, correct? Can you show us your net use command(s) that you're doing for the mapping? And any more detail specifics on the error(s) you get would help.
I would try creating two more AD accounts, one for each win 2012 machine to do their mappings - and see if you get the same issue.
If you don't get the same error, then I'd say this is more of an issue w/multiple connections- similar to what I just mentioned.