What happened to Olivia in the 2018 Lego Friends sets?
Many adjustments have been made for 2018. The five main characters all still exist, but they have been changed somewhat. This was brought up on the Lego Ambassador Network, and generated enough interest to get an official response from the Friends team in Billund:


  Every year we strive to innovate and make our products even better for
  children all over the world. A lot of effort has gone into the
  development of the LEGO characters and universes across our entire
  product range, so it makes us very happy  when changes are noticed.
  
  LEGO Friends in 2018 looks a bit different than what it has in the
  past. To be specific, the 5 main characters have a slightly different
  look but also the city and the citizens of Heartlake City have
  changed.
  
  But what are the reasons for the change?
  
  Firstly, we want to assure you that all 5 friends are still there in
  2018. Andrea, Mia, Emma, Stephanie and Olivia – they all still exist! Also the character’s main interests and personalities remain mainly
  unchanged.
  
  What you will see in 2018 is an evolution of LEGO Friends, a
  progression of the story and characters to make sure children get an
  even greater play experience. Every year more than a million children
  and parents reach out to us and share what they love and what they
  would like to change. In the LEGO Friends range children told us that
  they would like even more differentiated characters and also suggested
  improvements to Heartlake City.
  
  We always take great care to listen to input from children, and the
  LEGO Friends team has worked hard to make even more engaging and
  relevant experiences for children – and also make the LEGO Friends
  universe more true to the actual world children live in.
  
  Taking a starting point in reality, we’ve made the characters more
  diverse in their appearance and have added more depth to their
  personality. And of course, they still live in Heartlake City. But
  just as the characters have changed, Heartlake City has become more
  differentiated and rich. There are different districts and there are
  other citizens that play a more active role in the story.
  
  The changes we have made does not change the fact that previous and
  new LEGO Friends sets will cater for great building and play
  experience in 2018 and beyond, and we hope to inspire even more
  stories and play opportunities for children in the future.