Status: Fixed for next update.
When one loads a single level file, whether directly or from Test Mode, even though it says "NeoLemmix - Single Level" on the top of the window of the level, it still shows "Untitled Level Pack" on the taskbar (as shown in the circled part of the first picture) and when Alt+Tabbing.
I am unable to replicate this issue (on Windows 10). I've sent you a version by PM that might fix this; let me know if it does or not.
What version of Windows is that anyway? ??? I've never seen the Start Menu icon looking like it does in the screenshot, and I browsed through all the relevant screenshots in Wikipedia starting from Windows 95. All the ones before Windows 8 were "wavy", while the ones from Windows 8 onwards are straight trapezoids. That clam-looking Start Menu icon in your screenshot look like a custom thing? ??? With that in mind, are there other 3rd-party customizations/replacements of the Start Menu and/or Taskbar installed on that computer?
It's Windows 7, but with the Classic Menu Theme. That's Classic Shell, which brings up the old Windows XP-and-below style menu.
Anyway, the problem is fixed now.