Hello,
we noticed that these "extraClasses" are not included in the minified version of the used leaflet.awesome-markers
https://github.com/lennardv2/Leaflet.awesome-markers/blob/2.0/develop/dist/leaflet.awesome-markers.min.js
but in the unminified version
https://github.com/lennardv2/Leaflet.awesome-markers/blob/2.0/develop/dist/leaflet.awesome-markers.js
so we solved this by
adding the unminified version to the project
https://github.com/hefan/FieldtypeLeafletMapMarker/commit/9aeefed37b8065d71fc3a23ed80d9ee41eab974b
and just link this file in "MarkupLeafletMap.module" file
https://github.com/hefan/FieldtypeLeafletMapMarker/commit/0f5f563cdf2df725a32e1a3bbeb30a8790f4bd13