We use a lot of Lottie for different purposes in our product. Currently I export a video with white background and place it to Principle edit file. Problems occur when other objects play below Lottie file, or Lottie animation file goes above certain UI elements (e.g. we have a cheering explosion, elements of which are flying around and covering some body text.)
Unfortunately it is not possible to play a video with transparent background, hence my request to support Lottie with transparency.