Perfectly understandable you not being happy.
I have never be called Taff (that I can remember) but doubt it wouldn't bother me anyway.
That said, it wouldn't sit right with me calling someone Mick if he was Irish or Jock if he was Scottish, unless of course they were happy being called that.
Was in a Jack's company once and he was furious when someone called him Taff.
Funny as feck so it was