That does look like a long length of unsupported steel. I could see it being susceptible to bending, especially from an uneven load, as caused my a single painner. Looks to be made even worse by the disk brake mounts, canting the rack further back.

I would suggest fabricating your own mounting strap, I used a piece of 1/8th aluminium sheet, with a few holes drilled in in to mount my rack. It's much stiffer than before, and a lot less likely to bend.