This looks like it could be a Bug. The faces are not pulling back to the bends, even at a larger bend radius of 4 mm the faces still are not ending at the bends.
For now, unchecking the Reverse Thickness will let it unfold. Even with reverse thickness unchecked it looks like there is still a bit of the face protruding into the bend.
I took a couple of screen shots that highlight the issue. The first has the material to the outside and I increased the bend radius to 6 mm.
The second image has the material set to the inside and shows the two faces extending into the bend and each other.
Maybe Support can shed some light on this.