I ran into this transparency/AO problem as well.
The weird thing is that it appears to be related to the size of the transparent object. For example, AO worked fine on a character with transmapped eyelashes, but as soon as I threw a tree with transmapped leaves into the scene, AO was gone.
Disabling "visible in raytracing" does not help. I'll probably have to hide the tree, render with AO, unhide the tree, render again, and composit in Photoshop. Grr.

By the way,strand based hair is not treated as being transparent - AO works great when using strand based hair.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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