It seems there is a bug in SAB 12.6 that breaks the content menu. Once built the Contents Menu presents fine, but none of the links work. From diagnosing what may be causing this, it appears the target links are not being kept. 12.5 works fine.
In 12.6 - even after re-entering the link, it is not kept. I have both 12.5 and 12.6 running on my Mac and 12.5 shows the link; 12.6 does not.
This was also confirmed in that the build from Scriptoria (using 12.6) didn’t work, but the build from 12.5 - built locally and then sent to the reviewer does work.