As described throughout ISO 32000-2:2020, clause 9 "Text", handling of vertical text requires special attention to glyph origin and displacement vectors. If not handled correctly horizonal glyphs can be used in place of vertical glyphs, as illustrated by Figure 55:

VerticalText.pdf Renders Japanese vertical text using a non-embedded font with appropriate font and font descriptor entries for vertical writing. Because there is no embedded font in this PDF, a substitute font supporting vertical writing is required to be present.