This might be the most niche bug there is but here it goes: My computer was originally set to German as the default system language. Quite some time ago, I changed the system language to English (United States). Windows doesn't seem to be quite as thorough with that change as the weights of my installed fonts are still in German.
Now, the font weight selection in HitFilm (Pro) seems to rely on that font weight name rather than an enum value. Therefore, when I change the font weight on a text element nothing happens except when the font weight is (accidentally) in English:
My System specs: