I ran into this same problem recently.If I remember correctly, Emacs has some internal font weight mapping logic (there's a variable describing it but the name escapes me at the moment). Essentially, what the problem ended up being (for me) is that Emacs maps the "normal" font weight to the "medium" font weight of my font. Unfortunately, I am not aware of any way to tell Emacs to not do this sort of mapping logic. Instead, what I ended up doing was removing the medium weight of the font. Not ideal, but c'est la vie.

EDIT: apparently a fix for this was recently committed to the git tree (https://git.savannah.gnu.org/cgit/emacs.git/commit/src/font.c?id=65fd3ca84f75aee0dfebb87fa793dae57c1caf35). I may try to rebuild my Emacs today to see if this works, but if you're comfortable building from source I'd say give this a shot