I believe I found it.
In my research... wow... Microsoft really made a mess on how configuration profiles/policies set keys on the computer. They're all over the place and it's rather frustrating that excluding a this policy doesn't remove the policy-set from the system. Anyway...I digress.
"Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\AzureADAccount" with key "PreferredTenantDomainName" seems to be the primary culprit. Delete the key, then the domain appendage should be gone.
I'll list a few more locations that I've found in case removing just that doesn't work.
"Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\PolicyManager\providers" <----- this one is the fun one. You'll have to sort through UID folders and eventually you should find one that has "default > Device > Authentication" sub folders.
Hope this helps someone with my similar struggle. I'll most likely write a small script to adjust the key(s) and I may post it here.