Having spent a lot of time on your code, I'm 99% sure that a punctuation mark must precede the footnote calling number to make it correctly work.

"This is a sentence.1 This"
will work

"This is a word1 followed by the rest of the sentence."
will not work

I read many English text where the footnote calling number is located at the end of a sentence, just after the period. Maybe this is a rule of thumb in English typography. (Is it ?)
But in my native language, it is not ; footnote calling numbers can be anywhere in a sentence.