It can be done in 12 with the 'W Triangle fix 12' object.
http://archicad-talk.graphisoft.com/viewtopic.php?t=46933
I don't have 11 installed but if it can't be done there then I may be a bug in the library part.
Setting the offset to half of the width should work - that is all I did in 12.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11