I'm wondering if there is an imbalance in original old caliper one side and the new non original on the offending side, but I guess as the problem was there before, that blows that theory out of the water. I always replace items in pairs anyway, so starting with a known level playing field.
As Karl says check h/brake pads for even/uneven wear.
Also if you have a spring weight balance thingy - the type you weigh your holiday luggage with, or your catch with when fishing, with handbrake and both rear wheels off (chocks and stands) and both side studs in near same exact position, choose the same stud each side, hook the spring balance weight on and pull until the hubs just start to turn.
Note the weight pulling each side to see if there is more drag on one side than the other for the disc brakes, then with the handbrake partially engaged, repeat process for that.
It may give clues ?