The failure occurs because a part designed to be a strut (axial load) is loaded from the side (= beam) at a height where there's a big hole in it. Just replacing the part with another that has the same flaw invites a recurrence. To eliminate the weakness: turn the strut upside down, so the place where side loading happens doesn't have a hole. Given that users don't generally change rack height, one could cut off any excess length at the bottom if it interfered with something - although you were already using it at full length. So it sounds like good advice to anyone with these racks to invert the struts, and trim the bottoms if necessary.

When you say "replacing them", I guess you mean the panniers? You could eliminate the need for that by bolting the broken struts & the new ones you found - both too short for your application on their own - together, to make a strut that's long enough and solid at the point of side loading.