The failing call is arithmetically sound. Script capital A sits at U+1D49C, so the offset for B computes U+1D49D, and that slot is empty because Unicode declines to encode a character it has already encoded somewhere else [1][5][4]. Nothing throws. The string gets built and copied, and the renderer draws the box that means no glyph [6][11]. The letter that should have come out is not nearby: script B lives in Letterlike Symbols, U+2100 to U+214F, at least 111,438 code points below where the offset pointed [3][6].
That distance changes the shape of your strings as well as their contents. The dev.to write-up tells you that every character you are producing is astral-plane, two UTF-16 code units [12]. That holds for the arithmetic path. The eleven script substitutes sit below U+FFFF, one code unit each [5], so the code-unit length of a styled string now depends on which letters the user typed. Anything downstream that counts units against a limit is off by one per hole.
Bold has no holes, which is why the demo always works. In script, three of the five letters of "Hello" (H, e and o) come out of the exception table [7], and "World" comes out of the arithmetic clean [6]. Eleven holes across 52 script letters is about 21% of that alphabet [8], and none of it is visible to an eyeball, because an unassigned code point and a missing font draw the same box [11]. Hence the assertion the author recommends: run a pangram, filter the output against a set of reserved code points [9]. That set has 24 entries [2]. Eleven of them are the script holes [8]; the other 13 are one isolated point at U+1D455, five between U+1D506 and U+1D51D, and seven between U+1D53A and U+1D551 [3].
Adoption cost is small and permanent. One table per style, one assertion in CI, and a maintenance risk that is easy to name: a style added later whose holes row nobody wrote, passing a reserved set nobody updated [7][9]. The other transforms have the same shape. Flipping is substitution plus reversal, and the substitution table has to be an involution, because people paste flipped text back in and expect it to come home [15]. The correctness of one of these generators lives in its data, and the offset was only ever a compression of that data.
