It's fine for a monospaced font to contain an "fi" ligature (to represent Unicode U+FB01), but it absolutely should not contain a "liga" feature to form it from "f" and "i". I'd personally not even create a "dlig" (discretionary ligature).

Not surprising to see this, because the rules are quite different for non-monospaced fonts (almost all fonts should have a liga feature for fi, and all modern text layout engines should respect it).

Source: I've worked in this space.