I'm thinking this is a bug, depending on how you look at it. The default settings on ubuntu 12.04 using the virtualmin install script can cause restores to fail.
MySQL comes with the default setting of "max_allowed_packet = 1M". This causes some virtualmin restores to fail because some queries in the mysql backup are larger than 1M. I propose either raising the "max_allowed_packet" to something higher and/or forcing virtualmin to breakup the queries into segments smaller than 1M. I'd favor increasing the "max_allowed_packet".
To elaborate, this is a fresh install of Ubuntu 12.04, fully updated. No extra packages installed (no apache, no mysql). I then installed virtualmin with the install script. The mysql backups were made by the virtualmin backup process. The restore was attempted through the virtualmin gui.