The first image below is the result of /setblock -49 101 14 bell[attachment=double_wall]. It appears to be attached to only one of the adjacent walls. (I'm not positive the tracker will keep the same order, but hey, if it's reproducible you'll see for yourself.)

The second is the result of /setblock -49 101 14 bell[attachment=single_wall], and it appears attached to both adjacent walls.

These seem backward to me. I would expect "double wall" to mean "attached to two walls".