Webmention itself only contains the link to the mentioning page, nothing more, not even the date/time (it’s up to receiving party to record the date/time the mention was received, if they care). Surprise: same goes for trackpacks and pingbacks! It’s up to the receiving-end software to do the processing (like: go to the source of the webmention, take a snippet and store it for later presentation).

If your software fails you, it’s not always an indication that something is wrong with the protocol.