Can't reopen #1, so I'm making a new issue instead.
I ran into this issue as well. The fix mentioned in #1 (removing the hyphen.lc glyph) worked for me too (I had to remove it in a few places in FiraCode.glyphs).
This should be fixed in the main code, so I'm opening a new issue. I'm also curious why this bug happens and what the intended purpose of hyphen.lc is.