I just added a frame-mounted Ibera PakRak (disc-brake version) to my MTB. I noticed right after that the rotor rubs on the brake pads (didn't before), so I looked up how to adjust the calipers & messed with it a bit and now instead of a consistent rubbing noise, it's intermittent, and I noticed the rotor appears to wobble between the brake pads. If I remove the cargo rack from the frame (at least the mounting points on either side of the frame by the rear wheel), it stops wobbling - leading me to believe the extra inward pressure from the rack is pushing something every so slightly out of alignment. In more messing with the calipers I haven't be able to resolve the issue.

I've gotten it to the point where, while the rotor still appears to wobble between the pads, it doesn't rub against them. But braking with that setup will cause uneven wear over the long run, won't it?

The rotor isn't warped - it wasn't wobbly before, and nothing hit or even came in contact with it since installing the rack.
Tektro IO disc brakes
Ibera PakRak
all on a 2007 Raleigh Mojave 4.0

Is this a common problem with frame-mounted cargo racks on disc brake bikes?
Is there a solution for it?
(Still fairly new to having & maintaining a bike, so pardon if I overlooked something obvious).