Missing from disk is exactly that - the folder for the plugin is missing from the disk. It is already installed in the database, which is why the plugins overview cannot re-install it. The solution to 'Missing from disk' is simply to put the folder back on the disk (not reinstall, just put the missing, forgotten folder back in place). You would have copied it manually as part of your upgrade (except it was forgotten), so this is just reminding you to do what you should have done originally.
I'm not sure I agree with you that its a bug. It may be a feature request for the tracker that if a file is missing from disk, then there is a link that purges all reference to it from the database, but I would (and this is a personal opinion only) regard that as an improvement and not the fact that its not currently there as being a bug