I noticed the issue started to happen right after I changed a setting in File Explorer: Settings - Search (Tab) - under: "When searching non-indexed locations" - Uncheck: "Always search the filenames and contents..." Apply and OK.
That totally makes sense, because in order to include in search any file contents from a OneDrive folder, it needs have it locally to index it, otherwise it only needs the file name.
Hopefully reverting that will make Runtime Broker stop downloading OneDrive stuff.