the characterStyleOverrides doesn’t happen to have the same length of the characters when this have emoji characters.

from the docs: https://www.figma.com/developers/api#get-files-endpoint

characterStyleOverrides Number

Array with same number of elements as characters in text box, each element is a reference to the styleOverrideTable defined below and maps to the corresponding character in the characters field. Elements with value 0 have the default type style