The doesn’t have load_balancer set, which corresponds to the plan showing that the attributes are being changed to null. As to how those values were originally set I’m not sure, but this seems like a bug in the provider where it should be able to keep or ignore the defaults somehow.
I am guessing that the provider expects the user to always configure that if it’s in use, so the fix would probably be to add that missing portion of the aws_ecs_service to your configuration.