I grabbed the Byzantine empire, which is elective. The 3rd destroy condition is negative ("can't destroy if Gavelkind"). I recall destroying plenty of other titles.
Say what?
Say what?
I tried changing it to primogeniture, same story.
Could it have something to do with any (my primary) title being Gavelkind? I find that I have +30% demesne even if only my primary title is Gavelkind. It's kind of a mess, really, because having different crown/succession laws across your titles incurs every and all penalties (one title with free investiture = -30 Pope opinion. 1 title with primogeniture = -30 pretender opinion from other sons etc). This does seem to ring true for benefits too, like the demesne size.
This.You cannot destroy ANY titles if you hold even a single Gavelkind title. You need to change out of Gavelkind first.