So, after retrying both the stock B747 and stock A320, I can confirm the issue is the sim is not loading the correct ILS frequency for 28C. Instead it loads the ILS for 28R, despite me creating the flight plan from the World Map with ILS 28C approach.
Since it affects both aircraft, I assume this is just one of those many fundamental bugs that makes relatively new sim pilots like myself unnecessarily frustrated! Now I can go back and work on technique until I find the next bug that makes me pull out my hair.
Thanks for all the tips and suggestions above.