The issue is that the standard spread between the lower mounts for MTB & Gravel racks is 165mm. Older touring bike racks were designed to fit a 145 mm spread. I have an older 26" MTB and a Jamis Renegade, both with std flat metal drop outs, and the rear drop out distance on both is 165mm, as is the spread on the Roswheel Tour rack I initially ordered. The Haanjo requires a spread of 185mm. The Roswheel rack is made from stiff, light, hollow aluminum tubing and the drop out mounting feet are swappable and held in place by two small screws each. The nice rear light attachment bar and lower pannier mount side bars make the structure really rigid, so it takes a lot of force to flex it even 1 cm wider. Even if pulling it open an extra 2cm didn't damage the rack, the added force would probably cause the screws holding the mounting feet to fail over time.
The only reason I mentioned the thru axle situation is that some of the racks advertised to fit any disc frame use the thru axle mount, which I don't think is an option here.
I spoke to a Trek store who said the
Bontrager Rack will fit, but these don't have the side pannier bars, and one review said it was too narrow to fit a Trek FX disc. Messaged Diamondback, and no response yet.