@Eric C, I believe there is some missing information in your explanation.
That you time-stamp the FIT file with our local time seems to be immaterial. I used the “Time Adjustor” on this site – https://www.fitfiletools.com/#/top – to take my ride from Sunday and “push it back in time” to Saturday. So now, on Strava, my Sunday ride happens “before” my Saturday ride, but my weekly mileage is now correct. The time-stamp on the “fixed” file is “today, 6:04 PM” (where today is 4/27). So it would seem that Strava’s adjustments are independent of the file’s time-stamp, because I was able to place my ride where I wanted it even though the FIT file’s time-stamp is two days+ after the ride.
If the time-stamp is immaterial, then something else is happening, so please allow me two clarifying questions:
Do FIT files have a <time> field like GPX files?
If so, does Strava use those values directly, without adjustment?
If the answers are “Yes”, and “Yes”, then clearly the <time> field could be adjusted in the FIT file before it was uploaded to Strava.
If either answer is “No” can you please elaborate on what happens instead.
Thanks.