Version of InDesign? Operating system? Happen in one file? All files?
The default font for modern versions is "Minion Pro", but it seems you have a local override, or perhaps a character style, that sets just the weight to "Light".
You can easily find the fonts in use by clicking that button in the dialog that says "Find Font..", or alternatively close the dialog and call up "Find Font" in the Type menu. You can then look for occurrences and close the dialog so you can manually fix it, or select a replacement font and use "Change" or "Change All".
Yes, that is an incredibly annoying "feature" that is completely redundant!
Minion Pro Light does not even exist.
If you have somewhere defined Minion Pro Light it might have different reasons:
- You have applied Light (Manually or with Character Style) when a different font was given to the paragraph.
- You may have defined a Character Style with Light attribute, but without defining the font family. If this character style is applied to a paragraph or Paragraph Style with Minion Pro as font family it will try to apply the font family Minion Pro with the Light style which cannot exist. This could happen automatically with GREP Styles, Nested Styles, Line Styles, First Letter and automatich Bullets and numbers but also with automatic text creation with table of contents or cross references.
Nope, it helps to avoid expensive production problems. Therefore it is important to have this functionality.