In iOS 14.2 I'm seeing this in every video I play in WKWebKit. I'm thinking the most likely cause is a missing "Entitlement" in info.plist or missing CORS headers for some of the content, or unexpected MIME types for the subtitles.
platypus_dolphin
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution
From
Tierra del Fuego, Argentina