In DejaVu Sans and Serif, the interlinear annotation characters have zero-width empty glyphs. In DejaVu Sans Mono, they have spacing empty glyphs. They should have spacing visible glyphs: according to The Unicode Standard 9.0, ch. 5, p. 256, “The other notable set of exceptional format characters is the interlinear annotation characters: U+FFF9 INTERLINEAR ANNOTATION ANCHOR through U+FFFB INTERLINEAR ANNOTATION TERMINATOR. These annotation characters should have a visible glyph display for fallback rendering, because if they are simply not displayed, there is too much potential to misread the resulting displayed text.”