Orphaned pages not hidden in tree

I've found an issue that may be a bug, but may be the way Confluence works. If this is the case, then it's very odd behaviour to have as standard. (smile)

I'm trying to hide a page from the tree, so I've made it an orphan page.
If you open the top page of a space, which the tree is at its most minimal, then the orphaned page isn't visible.

However, if I click on a link in the tree that I have clicked on before, the orphaned page is now visible. (I can see the difference between a previously selected and non-selected link because they're different shades of blue.)

If I return to the top page, and then click on a link in the tree that I haven't clicked on before (ever...) the orphaned page isn't visible.

But (with the orphaned page now visible in the tree), if I click on a previously clicked link or one that hasn't been clicked before, the orphaned page remains visible.

I've checked and re-checked this, so I'm sure this is a reproduce-able problem.
We're running 5.9.9 server.