I did that and get no value from the bounding box min (tried to run through separate xyz too)
It can generate the correct bounding box for a collection instance but does not return values for min and max. It does work with regular objects.
What I’m trying to do is I want the ability to easily measure the size of collection instances because I have a lot of assets that were not made at real world scale and not having this thing for collection instances is super annoying:
if I scale a cube down to 0.1 and apply scale and then join that with the collection instance bounding box geometry it does work. It doesn’t work if I make the cube smaller than that. It doesn’t work with a plane or triangle or single vert or object containing no geometry.
Correction, it doesn’t work. It’s only reporting the values of the tiny cube, even though it’s clearly capable of creating a cube out of the bounding box data from the collection instance 
I can even set shade smooth and add subdivs on the cube generated from the bounding box data. It makes no sense that it won’t return the actual values for a collection instance.