Refresh in client is not helping here. We had the same issue and our Mendix contact proposed to create the whole page instance again and replace the current view by it. The user doesn’t really notice he/she has moved to a new instance of the page except for the TreeView is collapsed again.
I think this is a very hacky way, but we haven’t found a clean solution for it.
The TreeView widget needs some improvement I guess. :P
I am not familiar with the Tree View widget, but what you can do:
Instead of a commit only, you can use a Change Object action with both Commit and Refresh in client enabled.
Also, you can try a Change Object with Refresh in client enabled for the parent object.