In T293515 we added ParserOutput::addWarningMsg() to replace ParserOutput::addWarning() -- but we didn't add code to serialize this to the cache.
As such we can't yet use this information in postprocessing or other pipelines which could be retrieving fragments from the cache.
Once this is cached, ParserOutput::getWarnings() should generate the message text on the fly from the cached arguments, which will also help ensure that we don't split the cache on language.