I experience the same behavior on Windows.
I wouldn't say it's a bug. It looks like Dropbox shipped "Selective Sync" and "Smart sync" features without making the user understand what both do, and how to enable/disable one or the other.
I find it especially confusing that Selective Sync has its setting in Preferences, but Smart Sync is just a contextual menu when you right click on every folder in Finder/Explorer.
I think they'd benefit talking to their existing and paying users instead or trying to shove down Family plans and Signature down their throat.
Right now my Dropbox client is uploading or downloading 80 GB of files for hours, it's not clear and I don't know why, because the same files exist as full binaries on both sides. I wanted to avoid the Dropbox client shadiness by uploading my files directly to Dropbox web, but I've created a giant mess instead ...