I am not experienced with Wild Rose from out west, but can only speak of Multiflora we have out east, which responds most differently that most any wood I have dealt with. The Multifora worked best if you stripped the bark and thorns off right away and started green straightening while it dries over days. And time I let them dry without green straightening and did heat straightening they would take minor heat bends but no heavy bends. A heavy bend was likely to split with the grain at that location.
Other wood would split badly if the bark was removed right away but not the rose. Other woods took heat bending better, but not the rose.
Also on applying heat I have always used a 2" candle with bacon grease and an wooden arrow wrench. Grease the spot, heat the spot when it feels like it will burn you almost use the wrench to apply force for a few second until it is straight.