Alt-Tab'ing can actually lead to a multitude of problems with Leejunz. I for one have never experienced the missing (or "empty") health- and energybars, my game crashes though if a new round starts while I'm tabbed out. As far as I know, most of the problems can be connected to specific GPUs, resolutions, or display modes/settings. That's why it's especially important that you include your system specs and logfiles when reporting problems like these.