Data point, as someone who was in that exact situation 4 years ago:
The Delta FA wanted the infant in the car seat for take off and landing, even though the kid at that moment was “happier” in moms arms.
Could have been rogue FA. Could have been lesser known rule I was not aware of. Kid wasn’t as happy for takeoff and landing, but went to moms arms again to be comforted once we were up in the sky.
YMMV