Hey Marian!
Thanks so much for re-watching this scene for me!
If i put myself in Jack's shoes at that point that totally non-existent reaction feels absolutely unnatural to me. It is simply not plausible. A few moments earlier Jack is smiling into Ennis' hair, "4 years, damn", "redlining", "Brokeback got us good" - he's in an euphoric mood... I cannot see why he is not responding emotionally after Ennis' rejection at that point.
If you, too, start to argue with filmmakers' decisions, then i also tend to think that this lifeless Jack is a result of scene splitting. As mentioned, the scene in the book is all complete at the motel and Jack's reaction much more natural and emotional, while Ang Lee decided to split the scene and probably saved Jack's outburst for later, at the river.
So, it IS an imperfection...