Extensions can use ParserOutput::addSecondaryDataUpdate() to register a callback to be executed when saving the respective content. Typically this is used to track some aspects of the page content in extra database tables.
Since DataUpdate objects are generally not serializable, ParserOutput skips them during serialization (see ParserOutput::__sleep). This was no problem before, since the parser output would only go to the cache after it had been saved, and WikiPage would apply DataUpdates based on a "fresh" instance if ParserOutput.
ApiStashEdit changes this: now, the ParserOutput that WikiPage uses while saving the content may be coming from the cache. In that case, it would have silently lost any DataUpdates that an extension may have registered, effectively disabling the custom tracking of page content.