I created a very basic Trigger:

trigger HelloWorld on Lead (before update) {
    for (Lead L : Trigger.new) {
    L.FirstName = 'Hello';
    L.LastName  = 'World';
  }
}

When I create a new Lead record in Classic, it works as expected (ie, no change since it should fire only in Update) but when I create a new Lead record in Lightning, as soon as I save, the first and last name gets updated to Hello World.

This is a brand new developer Org and so there are no other triggers etc. Has anyone else run into this problem?