I have built many trailers on and off for the past 40 years. Over 30 years ago I used to build my own axles. Yes, they are built in jigs, but the welding process can draw the spindle to one side if you aren't careful. I always used Lincoln 7018 LH70 rod for the axles, axle mounts, coupler assembly and any critical area. A very small amount of warping or drawing of the steel in the welding process can make a significant amount of difference in the spindle alignment. I can assure you, most manufacturers won't take the time I would to make sure everything is "right". I quit making axles, because you can buy them cheaper than you can make them when the big boys buy the components and materials in such large quantities.
As far as a solution for those who are having axle bending issues, as stated above, go to the next size up in the axle rating, but you don't have to go higher in the spring rating. A heavier axle with a weight appropriate set of springs will give you a good ride and a stronger axle.