When you assign a reference to a form (or one of its controls) to an object variable which is stored outside the form module, you must set the variable to Nothing or the form won’t be completely unloaded. In other words, it will become invisible but will continue to take memory.

Note that this isn’t a bug, because it’s simply a consequence of the reference counting rules of COM. The only unexpected detail is that a reference to a control prevents the entire form from unloading, but this is reasonable because the control can’t live outside its parent form, so VB is forced to keep the form alive as well.