I've just experimented with this a bit more, and it seems Helen is right.
If you right-click a folder, and select 'Sync to this device', its contents will automatically sync from then on (new files will appear, not placeholders). And I can't see any way of changing this back, so once it is set, you are stuck that way.
So it looks like Resilio does remember, on a per-subfolder basis, whether it should sync the files, or sync the placeholders. It just doesn't let you see this setting or change it easily. So I guess what I'd like to see is something in the right-click menu that tells you what the setting is, and lets you choose.
Also, I think you should add a confirmation dialog to the 'Remove from all devices' and 'Remove from this device' context menu items. It is very easy to hit the wrong one when using a tablet, and not notice until the archive has emptied itself.