Is that new ?
So a Domain Admin can't grant SQL Admin access to another Windows Admin account, if the Domain Admin has not specifically been given SQL Admin permission at install time, or from an existing SQL Admin ?
Nope, it's old, and by design. I, personally, don't want my Domain Administrators having access to my SQL Server's (especially with
sysadmin privileges!!!) so I'm pleased it doesn't automatically grant them access as well.
If I did want my Domain Adminstrators to have access to the instance, I would create them, or their AD group, the needed
USER objects with the minimal amount of permissions they need; which would certainly not be
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.