In the Moodle mobile app on iOS video captions are not shown. In the browser and in the Moodle desktop app (macOS) the captions are shown.
In a client's WBT videos are handled with MediaElement.js http://www.mediaelementjs.com/. In the videos captions are used. The caption files are loaded via AJAX by the MediaElement library.
Cordova for iOS uses WKWebView to display the content. The Moodle app downloads the content to the device and serves the files locally. Due to iOS CORS restrictions the XHR.status result for local files which are requested via AJAX is always 0 - even if the XHR.responseText holds the correct caption text.
Ionic WKWebView page
Support serving local content with WKURLSchemeHandler
Discussion on https://github.com/apache/cordova-ios/issues/415