When a text in a QTextEdit is underlined with QTextCharFormat::SpellCheckUnderline, that underline vanishes for large font sizes (the minimal font size for it to vanish seems to be font-dependent). See attached minimal working example and the screenshot (left). This is reproducible on both Windows and Linux.

Curiously, QTextCharFormat::WaveUnderline seems to work, although with a few issues on Linux: for large font sizes, it is partially clipped at the bottom (screenshot (middle)), whereas for small font sizes it can overlap the text (screenshot (right)).