The usual problems with the ISC motor are
a) one of the two coils goes open circuit ie it burns out.
b) the ISC internal gears chew themselves up, likely causing a).

The issue is therefore why the ISC chews up the internal gears. The usual
reason is that there's a build up of carbon and fuel gum in the throttle
body that stresses and jams the ISC gears. This gunk has to be cleaned out.
Further, most of the carbon comes from the PCV valve that vents hot oil
vapour into the intake from the rocker cover. This oil vapour can become
excessive when the valve stem seals are old and not doing their job
adequately any more. Since your car is a 97, its not unlikely. Piston rIng
blowby is another source.

At a guess, the dealer tech has just replaced the ISC motor without doing an
adequate check (and clean) of the rest of the throttle body to ensure that
the ISC can adjust the speed freely. It may be that the new ISC is faulty,
but its unlikely. An unexpected failure (esspesially if the ISC gears are
again stripped) should result in a more detailed investigation.

Yes, its a warranty issue. Be polite to the dealer.