Slitherine

Siege expert does not work.
I am besieging the Romans of Taranto.
I have 13 siege value from units
and leader 1-1 siege expert +2 siege value.
I should have 2 siege value for leader +1 for 1 Atk and +2 for siege expert and I only have 3.

Hmm didn't note that, but in one siege where I used it the flavour text said something about using a huge ram to break down some wall, maybe that is the effect?

That happens when you play a regional decision. No regional decision played no such effect.