The "total fragmentation" option is, as far as I know, not enabled in the latest build. That is a feature which is supposed to better handle collisions between large equal sized bodies to get a more realistic behavior.
It is the same feature which will trigger fragmentation inside Roche limit.
In this video, a "roche limit" fragmentation is demonstrated near the end.
https://youtu.be/4Qr6c_0ngTQThe tech exists as is, and it will be included in an update soon.