That's interesting. Not sure how every trailer bumper is welded, but every camper I've had, the bumper was welded to the frame on each side. It would appear to me that mounting in the center would be the location allowing the most yield even though the bumper would twist along it's axis up to the frame welds where the steel temper would have changed due to heat affected zones, so I'm a little dubious.
I mounted mine on the right side as close to the frame welds as possible, and it's very rigid. Now, I haven't sat on the tire while traveling, but if it's rigid in my drive, then I expect little change when traveling.
2016 Ford F-350 Lariat CC, 4x4 SRW. B&W turnover with extender. 2015 CC Silverback 29RE, fitted with Reese Goose Box. Amateur Extra Class K5TS, 2016 nights camped 16