I came across this twitter thread during the last Facebook outage. Apparently something very wrong is happening there with their backups. I would definitely check similar cases. Confirmed by multiple people :/
I quote the author:
This is really weird. In #WhatsApp, I started to see messages that I know 100% that I deleted 2 days ago?! WTF is happening there? I think this is a really big violation of privacy! I see the messages from a month ago, with my disappearing messages setting turned on?! Gosh