Pretty sure Strava uses HR or RPE instead of TSS and IF. I rode the same exact indoor ride two weeks apart that took me something like 2 hours and the second ride was mere seconds faster than the first one, but the Strava relative effort was almost twice as high the second time because my HR was a zone higher throughout (had lost fitness fast due to being completely off the bike during the bad AQI event affecting the West Coast). My FTP was unchanged and I don't ever bother reporting RPE in my rides.

I find Strava's fitness metrics to be a joke, and not just because of that. Let my trial premium sub end and haven't missed it one bit.