In my zotero-texmacs-integration, when I set the CSL style to ISO-690, it outputs "ISBN" followed by a two-byte character, followed by the actual ISBN number. I'm taking the text it sends me, which is output via an output format specification that wraps the text as LaTeX "bbl", and transforming it into a TeXmacs tree with: (latex->texmacs (parse-latex str_text)).
In the result, I see, in red, <varspace>. When I catch the offending character in an Emacs terminal buffer, and run it through Guile's string->list, I get: (#\302 #\240); and (map char->integer (string->list " ")) yields (194 160). As hexadecimal, those are 0xC2 0xA0. That is UTF-8 encoded NO-BREAK SPACE.
What this tells me is that TeXmacs is converting the UTF-8 Unicode character into one of it's internal things, but instead of it being rendered as an actual non-breakable space, I see the string <varspace> in red letters. I see the same thing when I paste that character into a document.