Thanks Chris. I confirm this behavior, but I'm not sure its a bug.

The selection reflects only explicitly selected tags. The implicit selection is not visualized.

I think the behavior should be symmetric: Note that when you select 'open', 'open/important' is not selected. Then  why should 'open/important' be deselected when you deselect 'open'?

Have you tried pressing Ctrl+R? (Or right click on Tag sidebar , "Display related tags only"?)

Fixed in 1.8.5

Release in 1.8.5