When a variable font is onboarded to Google Fonts, we will provide instantiated static fonts for legacy applications. Thus, bear in mind Google’s API will automatically create all the 100x weights along the used range of the axis when producing static fonts, whether or not they are included in the designer's original design space.
https://github.com/googlefonts/gf-docs/tree/main/Spec#stat-table
According to Google Fonts specs:
So, a SemiBold static font is downloadable on https://fonts.google.com/specimen/JetBrains+Mono
Please add a SemiBold instance to the VF.
See also https://github.com/google/fonts/issues/3411