Not sure if this first part would be called a "bug" as much as simply anamolic.

Each song window INSIDE ModPlug's main window has three of its own buttons. A Resize button, a Restore button, and a close box. The ones we're concerned with are the Resize buttons and Restore buttons.
Default behavior is for ModPlug to open a song window at "Maximized", in which the child window occupies all the available ("client") space inside the main or "parent" window. As a side note, i've noticed that sometimes windows of tracks which were saved in previous versions opened in non-maximized, non-minimized child windows. In the Maximized state, the child windows buttons become "Minimize" and "Restore Down," which appear in their tooltips. Click on the Minimize button, which (in Windows XP's default theme at least) shows as a solid line, like an underscore, and the child window hides and the titlebar is reduced to a smaller size at the bottom left of the parent window. If you were to instead click on the "Restore Down" button, which looks like a pair of small open boxes, the child window shrinks to a default size (or the last dragged size) in the middle of the parent window (or wherever it was last dragged by the user).
In the "Minimized" state, the buttons change to "Restore Up" and "Maximize," and do what you might expect. In the child window's "Restored" state, which was the last default or custom dimensions of the window, you can either Minimize or Maximize the child window.
All this is fine, but one wonders WHY you would want to minimize a song window inside the parent window. It seems it's ideal under two scenarios: when you want to open multiple tabs for the same track ("MultiView"), or if you want to compare two different tracks, each in their own window.