Requests made by VisualEditor will implicitly contain Accept-Language header provided by the browser. The browser sets the Accept-Language to the preferred system language of the user, and for wikis that support language variants, we split the Varnish cache by variant. So, for example, if a person who has English as a default system language goes to edit Serbian wiki - he will most definitely get a cache miss.
More importantly, for some languages OS allows to select a variant, for example, zh-tw for Taiwan Chinese variant, so REST API will serve the content in a variant that after saving in VE will potentially create a dirty diff since the original wikitext was stored in a natural, mixed variant.
Until we start supporting visual editing in variants in Parsoid, VE needs to explicitly set the Accept-Language header to the page_language of the page being edited.