So let's review:
Nothing was changed yet you have burning coals falling off the grate, so far you have drastically reduced the max setting with little effect. As WNY said draft could be a key, also combustion air could be a key, as he said the Tri-burner setting is not what you want, but that wouldn't have changed on it's own so unless you were in the setup menu just before this happened that is unlikely the cause.
I would look at the combustion air, are the fans spinning at full speed? Maybe fines have restricted the air flow through the grate, you can remove the combustion fan and look in with a flashlight without shutting down the stove. Also are both grates behaving the same, or is one dumping more hot coals than the other. If the change was identical in both then I would look toward the draft issue, your pipe could be clogged with fly ash.
Hopes this gives you a few more ideas.