Availability Group Endpoints Windows Authentication Error
So i am assuming its not causing any issues yet, but regardless i would like to know if its something i can ignore in my scenario, or fix. 


Looks like since everything seems to be configured properly (I can't see the domain account information [which is redacted as it should be :) ]) so I wouldn't worry about it. If this happens on multiple servers in the environment then it may require further investigation but overall everything you posted seems in line with a normal configured system.

I traced this down to a call IsValidDomainUserForWinAuthentication used in SSMS which, for whatever reason, is returning false. Looking at the logic, it's checking a few various items, but the one that stands out is the assumption on how the service account name is configured (allowable characters) and that calls to LookupAccountName complete successfully. It would take a time travel trace or full memory dump of SSMS (when the message box is shown) to really investigate further which I don't believe is warranted at this time.