At least when a drive is unmounted or a watchfolder can't be found, it does remove those videos from the library (which, incidentally, is a behavior we'd like to change in the future, but anyway), but they don't disappear from Mynda until it's closed and reopened. This does not happen when manually removing a watchfolder. Unknown if it happens when an individual video is removed from an existing watchfolder.
At least when a drive is unmounted or a watchfolder can't be found, it does remove those videos from the library (which, incidentally, is a behavior we'd like to change in the future, but anyway), but they don't disappear from Mynda until it's closed and reopened. This does not happen when manually removing a watchfolder. Unknown if it happens when an individual video is removed from an existing watchfolder.