Hey everyone! I’m the same guy of the previous reply, but writing from my personal account.
For anyone having issues with fonts, I found a workaround. Albeit a weird one. The best part of this is that it works with FontBase. 
- Create a new folder directly in your C: unit for your fonts. Name it however you’d like. Move your desired fonts there.
- Open FontBase, set your root folder to the one from the previous step. Example: C:\FontBase
- Activate your fonts. Keep the program open.
- Navigate to C:\Users\YourUser\AppData\Roaming\Figma
- Delete files “font_cache.json”, “desktop_state.json”, and “settings.json”. Don’t worry, these will be created again when you open figma.
- Open Figma
- Your fonts should appear when you search for them, although not on the Installed by You section
- Done! I tried restarting and fonts still appear, as long as FontBase is open of course.
My guess is that by having a more direct path, Figma can locate your custom fonts more easily. Normally fonts are in C:\Windows\Fonts which isn’t a long path if you ask me, but maybe there’s a permission thing going on for not-system fonts.
Hope it helps.