If all is okay, You must check in Preferences>section Formats/codecs installed and here you will see if x264 is not only installed (like with synaptic) but seeing and using too by MLT and so by consequence by Openshot.
If all is okay, You must check in Preferences>section Formats/codecs installed and here you will see if x264 is not only installed (like with synaptic) but seeing and using too by MLT and so by consequence by Openshot.