Because at the 1:47:59 mark you can see through the robe (side profile when he's looking out the door and drops the cup filled with drano), and a few seconds later when she kisses him, the robe is tight across his back, no barb wire bulges (and there was a lot of barb wire to begin with), the robe is flat across his back.
The director may very well have intended for the audience to believe Toller was wearing the barb wire under his robe for the entire sequence, kissing included, that was what was implied (shot of Toller barbing himself bloody, flip back to church event, then flip back to Toller buttoning his robe), but the kissing shot, the barbed wire was gone, when she pressed firmly down on his back and held him, no bulging barb, no blood, flat robe across back.
Maybe they filmed the scene with the wires on and it didn't look right, or it physically hurt Mr. Hawke to wear the barb wire for any length of time, and they filmed the kiss without the wires.
Sorry, don't see it. Scoured the scene and at no time can you see beyond the robe. The bulges in the robe are always visible in the front, all the way up to the kiss. Since there should be more blood on the robe, I took it as a Goof or just part of the fantasy. If one believes it was a fantasy.