yeah i get that a lot too. seems like when i first start driving, the replay records fine, but then if i go to the garage and then go back to the track, from there on nothing gets recorded until i restart the session.
Yes, if you pit in it's interrupts the recording. Basically the only way round it is to restart your single player game each time you make a setup change. That's why you'll find Hotlap mode restarts when you do the same.
There's probably a reason for it, but I don't know what it is