Ivo's note: this was already fixed so code below is not necessary.
Leaflet 1.4 CSS codes contain serious BUG that we have fixed in our CSS build. So please do not include Leaflet's CSS to your html.
Edited by ivo
Thanks for all your efforts updating the API!
I found that, after switching to Leaflet v1.4.0, I now need to reference the Leaflet CSS, otherwise the URLs of marker icons are wrong, and the markers do not render correctly. I just needed to add one line in the header of my page: < link rel="stylesheet" href="https://firstname.lastname@example.org/dist/leaflet.css" >