As posted earlier if Darth isn't a name but an assumed title Obi-Wan using it becomes a dig.
Like Han's "Buddy" and Mace's "My Lord".
Even as an assumed name it's still a dig.
He isn't "Anakin" and he isn't "Lord Vader", he is "Darth".
As friend or worthy foe he would command respect but as the turncoat traitor Kenobi sees him as not deserving recognition of rank (even an enemy rank).
Well that's the way I've read it since 1980ish.