OP I know what the manual says...but I offered a theory in an effort to help. Here are the facts: Lower octane means lower resistance to predetonation (if you didn't know this already, google it). When you are in EcoPro the car runs a more lean map to get better economy. So your car runs as lean as possible which puts it as close as possible to the limit of detonation. When you accelerate, the computer senses knock and proactively retards timing and makes the mixture richer. But it can only go so far...if knock is bad enough, the computer will go into a limp mode to prevent damage.
So while you CAN run 89 octane, and under most circumstances it will be ok, your car will be more prone to knock when you do. Personally I would never put 89 octane in a forced induction car. Some do...I would never.
Now if ELK has some facts to show this is rubbish, or that your issue had nothing to do with octane, it would be great to hear them!
Again I am only offering a theory. Unless you get the codes read by the dealer all we can do is speculate. But you can test my theory - run 91 or higher and try to duplicate the fault. Then run a few tanks of 89 and try to duplicate the fault. See what happens.