Kind of weird as I thought I would follow the Roll20 guidelines by replacing the underscores with spaces upon submission. If you want, I can give you an itch.io key where you can download (and replace) the status icons with files having underscores in their names, by writing me a mail at info@stevencolling.com. What do you think?
I don't use Roll20 macros myself, but as a quick thought, did you try to use quotation marks around the file name? Like:
!token-mod --set statusmarkers|!"18 blinded"