Get a qr and drop the whole post when doing big descents! i doubt it’s anything to do with your suspension got to love this forum sometimes.
You’re getting bucked by the saddle hitting your ass which you’re not used to. So the saddle is too high.
You doubt its anything to do with his suspension, instead your suggesting its just his saddle is now somehow too high, despite it being the same seatpost and him not dropping it by QR in his previous frame? No sorry you’ll have to explain that one to me.
OP, if you’ve not set your rear suspension you really need to look at that. However the G150 will be quite different in geometry than your smaller older orange, its likely the reach to the bars is much more, so dropping behind the saddle now requires a stretch. This new long trend requires a different riding style and it takes time to get used to it and get the confidence. Its more about getting the body low and keeping central (to make sure the front wheel still has the same amount of weight over it as the older shorter bikes).
Personally im struggling to get used to this type of geo. If I consciously think about it im fine (diving into a corner at speed while leaning further forward than I am used to feels weird and a little scary), however get surprised by something and to lean back tends to be the natural reaction and this can make things worse and as you say, you can get hit by the saddle.