If the parent folder is filtered out, everything below it is as well.
If you click the hidden folder count icon (the one which has a ghosted folder icons and "0" in your screenshot), that'll turn off folder filtering so all the folders remain there, while the typed filter is applied to all the files.
This logic is different than Flat View... it's not an expected result I don't think. Why would I want to expand a folder, and then not have filter apply to that?
I see yes that I can click to Show All Folders after I select the filter, but that's an extra step I'll never rmember to do. Perhaps there can (or is) a setting to alter this behavior so it's the same as when in Flat View?
The advantage of this over Flat View, is I may only want to filter a particular folder, or two.
That was displaying outside the width of lister, so I couldn't see it. It would seem "Show everything" could be shortened to "ALL" and since the flat view one only appears when in Flat View... that can be shortened to "Filter folders".
I tried toggling the advanced bool, and turning Flat View on and off, it makes no difference. It think it's obsolete now.
"Show Everything" is the name of a specific mode/function within Opus, so renaming that checkbox would confuse things. It also affects more than just the settings in the Filter Bar.
The Advanced bool setting still has an effect, but it won't affect existing windows/tabs, only ones that are opened afterwards, since they lock-in their filter settings after opening. (I thought it didn't do anything as well, at first, but then checked the code and realised what was happening.)
On a more general note, we're thinking about changing how filtering works with nested folders (both expanded and flat-view), but not in the very near future.