Release 6 ships source-less pre-minified file (which is problematic from Open Source compliance and best practice perspectives): ui/js/vendors/Leaflet/Leaflet/leaflet.js
Minified code is effectively an equivalent of pre-compiled binary: un-readable, non-modifiable and non-distributable in Debian.
It is also a bad practice to commit binaries to VCS.
Please replace minified file with its original (non-minified) source.
The origin of the file if probably this (although I could not confirm because minified file have no version number):
Read more in https://wiki.debian.org/onlyjob/no-minification