I can confirm that Physics changes when in an online race. The same thing was one the GT6 to. Why it is, I do not know??
THIS! TL;DR warning. I was about to make a thread on the topic
can't believe search turned a result!
This was something that was very apparent in GT6 online racing. I'm definitely feeling it again in GT Sport. I felt it in the closed beta and made some comments, the dev team was like "eh not sure."
I'm speculating here of course, but I believe it has to do with other cars being present on track vs. when they are not, and especially multiple cars being in proximity to one-another on track. I call it a lag. More local congestion = more lag. So it's most prominent in the first lap, when congestion is highest and you are least used to it.
In GT6, it was all the same story. In practice prior to race start, I'd be braking at certain points, car behaving one way, achieving certain times. In the race, I'd have to dial everything back 10% or more, at least for the first lap until the pack thinned out and I put some distance on people. Obvious first comment on the theory is "drafting," but this absolutely happened when I would be on pole in P1 going into T1. I have vivid memory of leading a race at Nurb GP from the start, having trouble putting a gap on P2 and by T7 asking "how much longer until this lag starts to ease up?" The car was just sloshing around like a whale.
It might have to do with network latency, but other games (for me, Assetto Corsa) don't seem to have the issue. My ping is always low, usually 40-50ms. It could still be lag related - where other racers' higher latency takes a toll on everyone somehow? It could be hardware-based lag, who knows? Again, totally speculating but I suspect it's a part of the core game functionality and probably not solvable. All I can say for sure is that something is there, and it manifests as a general feeling of the car being a lot heavier and less grippy when among traffic.
This has cost me the lead twice in GT Sport - overcooking T1 and squandering pole position - even though I KNEW it could happen and went in extra careful. I'm used to cold tires into T1 from Assetto Corsa, so I know how to handle that - this is totally different.
It might also have earned me a win, when in one race I accidentally bombed into T1 way deeper than I ever normally would and went from 4th to 2nd. Took first by T4 and never looked back. Fortunately I didn't make contact or otherwise look like I was being a bad driver