Sitting on a trainer isn't the same as riding outside, your body and the bike interacts differently. Both are far more dynamic on the road/trails, others will have more saddle specific comments, I am sure

Cut outs aren't the same, they are all different. Some cutouts are longer, some are wider, some rails have more "give". Have you tried Knotted's cut-out test? It is here on TE, sadly I don't have time to go look for it. I learned the hard way that while I really did/do need a cut-out, there is still some searching to be done to find the one that your body likes.