Is EWS Managed API 2.0 GetFolder error response returning "ErrorInternalServerError" instead of "ErrorNonExistentMailbox"

Hi , 

I have been using EWS Managed API 2.0 for Exchange Online's mail operations.  Starting from 28-July , the "GetFolder"  request of unlicensed Office365 users started to return "ErrorInternalServerError"  instead of previously "ErrorNonExistentMailbox" , but certain amount of unlicensed users' error message still remain "ErrorNonExistentMailbox".  

May I know that is the error response of "ErrorNonExistentMailbox" going be replaced by "ErrorInternalServerError"?  Or is Exchange Online doing some upgrade that causes this?

Thanks