This is actually an interesting discovery and also has an implication. Previously WhatsApp would store undownloaded media on its servers for up to 30 days. This means that if you have yet to download it, say you’re overseas and have no internet or if your phone is getting fixed, not to worry as you’ll be able to download them when you get back on, after which said files will be removed from WhatsApp’s servers.
However with this change, this means that WhatsApp stores your media files on its servers even after it has been downloaded. It is unclear as to how long these files will be stored for, but according to WABetaInfo’s testing, they were able to download old files from as far back as 3 months, suggesting that WhatsApp could be storing your old media files for at least as long as that, if not possibly longer.
While we’re sure that some might be concerned about WhatsApp keeping your files on its servers for extended periods of time, note that WhatsApp does feature end-to-end encryption so there’s that. Also this seems to be a feature only available for Android and not for iOS users, at least not yet.