As documented in "File and Folders View",I've never tried it before, but it doesn't seem to work - at least not in the way that I tried it.
I created the folders: 'C:\QR Test', and 'C:\QR Test\Subfolder'
Set 'C:\QR Test' as a Quick Recovery folder in a new sandbox that I created.
Immediate Recovery is off.

Saved a sandboxed Notepad document to 'C:\QR Test'.
Saved a sandboxed Notepad document to the 'Subfolder', also.

In Control's "File and Folders View', expanded the list of "Quick Recovery Folders", right-clicked on 'Subfolder' and chose "Remove Folder From Quick Recovery".

After a window refresh, the folder 'Subfolder' and it's file were still listed under "Quick Recovery Folders".
A manual 'Quick Recovery' or a 'Delete Contents' still offers the 'Subfolder' and its file for Recovery - and the files are then recovered.

When you add a folder as Quick Recovery, everything below it (that means all files AND all folders) are considered eligible for quick recovery.

So really the only problem (although a minor one) is that Sandboxie lets you remove 'Subfolder' momentarily, even though it was never added as a Quick Recovery folder, and even though its parent folder IS set as a Quick Recovery folder.

I've always considered sub-folders as being included whenever a folder is designated as a Quick Recovery folder.
So I wasn't surprised that the files in the sub-folder were offered up for Recovery, except that the sub-folder isn't removed as a Quick Recovery folder - even "momentarily", from what I can see, after using this context menu command.

By "momentarily" I meant that it does momentarily disappear from the display, that's all. But I guess it's not even that: It just collapses the folder tree, and when you expand it again, the subfolder is still there.

This issue should be addressed in version 3.35.22. There will not be a use the command "Remove from Quick Recovery" on sub folders of quick recovery folders.