I've think I might have that problem.

The following figures are rough (within a couple of points).

I've tried a regular Long Bow
Max damage: 99
After Strength (94) and Augment Damage (5) modifiers: 150

I've tried a regular Crossbow:
Max damage: 65
After Strength (94), Augment Damage (5), and Crossbow Expertise (5): 116

I notice, exactly 36 damage points difference between before and after in both of them. Where has the damage bonus from the Crossbow Expertise gone?

Larian, can you please help us! I'd rather not edit my save file to convert those skill points into the Bow.