This is the most common culprit. Many amateur font designers use free software like FontForge or BirdFont but forget to fill out the font’s metadata. When the software asks for the "Preferred Family Name," the designer leaves it blank or accepts the default "New Font."
What happens? Font substitution. The operating system realizes the font you selected is missing the required glyphs, so it pulls those specific missing characters from a fallback font (usually Segoe UI on Windows or Lucida Grande on Mac). The result is a horrific Frankenstein text where your uppercase letters look cool, but your lowercase letters look like a boring system font. Font Substitution Will Occur Dafont