I’m booking flights to Pittsburgh from Dublin through Air France for next year and when selecting Economy Light on the app, it allows me to select seats for Dublin to Atlanta (a Delta codeshare flight) and same on the way back but not for the domestic US flights from ATL to PIT.

Is this a glitch or is that how Air France works? I don’t want to book the Economy Light fare and select seats just to have them null and void as you can’t pick seats on Delta’s Economy Basic.

EDIT: I’m able to book them free of charge. I’m an explorer level Flying Blue member.