Question Can't use local volume post-processing with URP and Cinemachine?

Using URP and Cinemachine, Im finding that local volumes either affect nothing, or everything. (No effect with blend=0, once blend gets to about 10 (my camera distance, I assume) it affects everything.

I'm trying to create a rectangle that can be moved around, making only what it covers desaturated. My URP post processing is already set up and working with a global volume (bloom only). I add a volume to an object, set it to local, add a profile and an override...everything looks like it should work, as far as I can tell. But at blend 0 nothing happens, and as I raise blend it suddenly affects the entire scene.

As often happens, I may just be misunderstanding what is being said in some forums, such as:
https://forum.unity.com/threads/how-to-use-post-processing-on-a-specific-game-object.1409024/

Am I reading this wrong, or is it indeed not so simple to use local volume effects when cinemachine is being used?