After more testing at different DPI settings on two monitors, it does not look like this is currently solveable. The issue appears to be due to incomplete Windows support for MDI windows when the DPI per monitor options in the manifest file are enabled. When these are enabled, Windows 10 is sizing the height of the non-client area/menubar/min/max/close buttons, which are beyond mIRC's control, differently depending on whether a client window is maximized/restored. For now, I will be reverting to the v7.52 manifest settings and will look into this again later in the hope that Microsoft fixes it in an update.