FEN includes ep square when en passant is not a legal move.

[JSLigon]

After the following moves, the en passant capture is not a legal move because it would put black's king in check, but the "Copy FEN" command gives the ep square as f3. Compare with the FEN for this position on lichess which correctly (in my opinion) gives no en passant square.

1. e4 e5 2. Nf3 Nc6 3. Bb5 f5 4. Nc3 fxe4 5. Nxe4 d5 6. Nxe5 dxe4 7. Nxc6 Qg5 8. Qe2 Nf6 9. Nxa7+ Bd7 10. Bxd7+ Nxd7 11. f4