Seeing as I am not that much of an expert on this topic I was waiting for someone else to chime in with an explanation. But seeing as there has been no response yet I thought I would put up my 2c worth.
I think part of the problem is the short distance between your departure and destination airports. If you take a look at the Arrivals and Approaches for KSTS, this is what you see:
From this is is clear that O69 is closer to KSTS than the entry points for all the runway 32 Arrivals. In your case, what the MSFS flight planner did was to not use any Arrival and just use the the LUSEE approach. But it did indicate that you were supposed to use Vectors to Final - the “VTF- KSTS - ILS32” line in your flight plan. So your flight plan basically had a gap there that you had to fill with assumed vectors to final.
Now for an interesting quirk: I made up the basic flight plan in LittleNavMap with no Arrival and just the ILS32 Approach. Imported that into MSFS to see what the simulator did. As an alternative I also created your flight plan in the MSFS flight planner. In both cases the FP looked like this in the flight planner:
Now I cannot recall which was which, but in the one case it came up with a flight plan identical to yours in the aircraft and in the other case it came up with this:
So it decided to insert the SGD arrival!
Neither of the FP’s in the aircraft was ideal, so what I did is just to delete everything up to the LUSEE IAF and then inserted a “Direct to” leg from O69 to LUSEE. This is what that looked like:
Not sure what that little white “curved tail” is at LUSEE but it did not affect anything in the flight. On track after takeoff.
Just before LUSEE, still on GPS. Was quite the task to get to 3,000ft before LUSEE!
On the localiser & GS before I started slowing down.
Followed the ILS perfectly. On short final
Unfortunately I have no idea if what we see happening here is realistic for the real life system or not. But it does work if you do what it takes.
EDIT: Oops, I was mistaken. Still 1920 x 1080 And BTW: have they again reduced the image upload size? I used to be able to upload 1920 x 1080 images but these got downsized to 690 x 388, which is pretty silly.