I don't own them but I'd say not normal in general, but these particular steps may have some flex that happens because of a subpar design. Just a guess. Double check they are installed correctly and torqued.
I've not heard many outright complaints about gorhino but I also don't hear about how strong and tough their stuff is either.
The only side steps I ever owned had virtually zero flex.
I had those running boards and removed them. They were delivered damaged/scratched and I was experiencing the same issue reported in another post on this site about the poor fitment of the brackets. GoRhino has acknowledged that the steps were delivered damaged and brackets defective. They have agreed to take them back, but I am still waiting for pick-up to be arranged and they (GoRhino and Stage3Motorsports) have since gone silent on me. During the short time that I did have the RB20s installed, they were barely holding on because of the brackets and yes, the running boards do flex when stepping onto them. I think this is a case of "you get what you pay for". They are thin, sheet metal, bent into a running board, but they were less than $500. I ordered the Zroadz instead. They were more than double the price, but they are at an entire different level of quality. The fitment was spot on and they are very well built. The Zroadz also hide the seam/weld on the lower portion of the Bronco very well. I am much happier with the Zroadz..