Some modules are only loaded if the content requires it. If such content is added in your edit, the module won't be available when you do an async show preview from the save dialog.
Related Gerrit Patches:
|mediawiki/extensions/VisualEditor : master||Load modules for preview|
Yeah... Since we use Parsoid instead of the MW parser for this, it might take a bit of guesswork based on what we get back. The MW api.php action=parse gives a list of the ResourceLoader modules used on the page - Parsoid doesn't.