Every so often, someone pastes flipped text and one or two characters show up as a small rectangle (□) instead of the letter they expect. That's not a bug in the text itself — it's a font problem on the device reading it.
Flipped text isn't a special font; it's made of real Unicode characters, many pulled from the International Phonetic Alphabet and mathematical symbol blocks rather than the basic Latin alphabet. Every device ships with a default font, and that font has to include a matching glyph for every Unicode character it displays. Mainstream fonts on modern phones and browsers cover these ranges well, but older devices, some in-app browsers, and a handful of custom keyboard apps ship with trimmed-down fonts that skip the rarer ranges. When the font has no glyph for a character, the system draws a generic placeholder box instead — that's the "tofu" box you're seeing.
Flipped capital letters and a few punctuation marks are pulled from less common Unicode blocks and are the most likely to show as boxes on an older or lighter-weight font. Flipped lowercase letters are pulled from more widely supported ranges and rarely have this problem.
If you're sending flipped text to someone and don't know their device, favor lowercase over uppercase in the line you flip — lowercase renders more reliably across older phones and email clients. If a friend reports boxes, the fix isn't on your end; it's their device's font falling back, and switching apps (for instance, viewing the same message in a browser instead of a lightweight app) often resolves it immediately.