Compatibility of Tiagra 4700 shifter with MTB front derailleur
With doubles this sort of setup can usually be made to work reasonably well. There's at least one bike I know of on the market with the exact setup you describe, the 2019 Salsa Fargo Tiagra, and it does work. Putting a triple STI on a mountain front triple crank and derailleur is where it gets bad.