Galvanic corrosion between alloy and carbon is a common problem for seatposts.It's one of the reasons carbon assembly paste exists. It also happens on bars (as shown above) and I've seen it on stems, too.

OP, what is the stack limit on that fork. You have a lot. Regardless of whether this contributed to the issue, I would strongly suggest going with the higher rise stem and fewer spacers also as shown above.

The stem failed along where I would expect to see a riser from welding on the tabs. Without the stem in my hands I would guess that it it fatigue from over torquing the stem bolts plus the added stresses of supporting more of the rider's weight due to the use of arrow bars with the large number of spacers being a slight but additional factor.

Since you need to replace the stem regardless, it would be prudent to discuss addressing this with your mechanic.