Did the bank know all along the term would have to be changed?
I would agree with the mortgage adviser in general that this case would normally retain the tracker, it's the term extension that is causing the problem as obviously the 9 remaining years of tracker rate the bank were already going to have to go with is now being changed to expecting them to provide a tracker for a much longer term, possibly losing them money depending on what the future holds!
I can see why they wouldn't be too keen, even had you not separated there is no reason why they would have extended the existing tracker should you both have wanted to.
Can they leave the you tracker on the new term but just for the 9 yrs and revert to SVR after that, would that be an option?