What you were expecting to happen, and what actually happened?

Hero equiped with new weapon crescendo which does 5 true damage to all. Brian the lucky is supposed to boost the true damage by 7 which should make him do 12 true damage to all. But instead only boosts him to 9 true damage total.

As I recall seeing in the announcement thread that Crescendo gets one damage added for every two magic points, so what you describe seem to be correct behavior:

The formula for Crescendo is DAMAGE = 2 + MAGIC/2 (rounded down).

So in your case: originally it was 5 true damage, meaning magic was 6 (2+6/2=5)
Buffed by Brian this makes 2+(6+7)/2=8 (rounded down). So there is something wrong, but in your favour I think.

There was nothing wrong. I assume magic was 7 (which rounds down to 5 damage based on said formula), after adding 7 magic totals 14, which leads to exactly 9 damage.