Saw a tweet on this but wanted to confirm it myself.
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If you close the game from the PS5 home screen while in an endurance race, instead of exiting the event (Retire), it remembers your progress and gives you the option to resume.
It also allows you to do anything else in the game while remembering your progress for later. To test that, I did another Power Pack race, and a regular World Circuits race. The option to "Resume from Progress Data" is still there.
Upon resuming, you are put into a rolling start. Gaps aren't remembered, however cars that were a lap or more down remain such. In that sense, it's a bit like closing the game causes a Safety Car, bunching cars up but not letting anyone get their lap(s) back.
Fuel depletion is remembered, however tyre wear isn't, though the compound you were on is.
Weather radar patterns are also remembered.
This is a guess from the testing I've done, as well as the tweet I saw originally, but it
appears to save progress every 3 laps.
So lap 3, 6, 9, 12 etc. Multiples of 3.